It depends on what you mean about "subtle underlying, seemingly unidentifiable cause of death". In fact, if there was a huge catastrophy like your mentioned asteroid impact then it won't kill all victims of a mass extinction at the same time. Due to famines and stress, diseases can spread owing to weakend immune system, while the forced migration owing to climatic changes into other territories will repress the primary animals of these regions. As you can see, mass extinctions have one big event (maybe out of the sudden or insidious over a time span) which effects the life in many different ways, therefore you have a combination of some lethal events. Maybe this article could be interesting for you:

John E. A. Marshall, Jon Lakin, Ian Troth, Sarah M. Wallace-Johnson. UV-B radiation was the Devonian-Carboniferous boundary terrestrial extinction kill mechanism. Science Advances, 2020; 6 (22): eaba0768 DOI: 10.1126/sciadv.aba0768

«Non sunt entia multiplicanda praeter necessitatem» ( William of Occam, 1285–1349 )

In fact, this is a very important issue, all the interpretations goes for some extraterrestrial factors or huge geological catastrophes. In my opinion, the only limitation to not think about a viral catastrophe, is the distribution of the animal reigns and their density

There are many different assumptions about the causes of this extinction. The vast majority of paleontologists agreed that the extinction at the turn of the Mesozoic and Cenozoic, as well as all other major extinctions, was the result of competition and displacement of some groups of organisms by more progressive others, changes in plant communities, eustatic fluctuations in ocean levels and increasing climatic instability, as well as extremely powerful manifestations of volcanism, especially explosive.

Some scientists (LS Davitashvili and others) believe that all the changes in the organic world that have taken place in the history of the Earth depend on the emergence of new groups of organisms that have supplanted others, less perfect. For example, ammonites with their bulky hydrostatic apparatus (hollow, multi-celled shell) could displace, killing a large number of their young, much more mobile bony fish. Small mammals that devoured their eggs could play a fatal role in the fate of dinosaurs.

Other scientists believe that the cause of all the sudden changes in the development of living organisms on Earth is a change in the intensity of cosmic gamma radiation. The latter led to a radical change in the structure of the chromosomes of the cell nucleus, and hence to a change in the properties of the organism itself (spontaneous mutation). Scientists IS Shklovsky and VI Krasovsky believes (1957) that the extinction of giant lizards at the end of the Cretaceous period was due to the explosion of a supernova, which increased the level of cosmic gamma radiation in the Earth's environment and giant lizards became extinct immediately and everywhere on Earth. It is thought that this increase caused the appearance of too thick a dinosaur egg shell and the embryos could not hatch (many finds of fossilized eggs with dinosaur embryos). But rising levels of cosmic radiation could not be detrimental to all organisms; for many other groups, it may have contributed to their rapid flourishing through increased genetic mutations.

It is possible that giant reptiles (reptiles) by the end of the Cretaceous were in the third stage of development - aging (the first two - the emergence and flowering). They have reached their stage of aging and have exhausted their life potential and have not been able to adapt to new living conditions.

Scientists from the University of California (L. Alvarez et al., 1979) have shown that at the turn of the Mesozoic and Cenozoic in some areas of Italy and Denmark there are geochemical anomalies of iridium in the boundary layers of clays. It is believed that this heavy metal is of cosmic origin, because all meteorites, compared to terrestrial rocks, have high concentrations of iridium. Scientists have suggested that the mass extinction at the boundary between the Mesozoic and Cenozoic was caused by the collision of the Earth with an asteroid, the diameter of which could be 10-15 km. The energy of the explosion was supposed to reach 1,030 erg, which is much higher than the energy that reached the earth's surface after the supernova eruption. As a result of a powerful explosion or, most likely, a series of explosions, because it is assumed that the asteroid split into pieces when entering the Earth's atmosphere, and the mass of Earth's matter, hundreds of times the mass of the cosmic body and turned into dust, was thrown into the atmosphere. The dust remained in the atmosphere for a long time, which greatly reduced the transparency of the atmosphere and disturbed the heat balance. Using computer simulations, scientists have calculated that ⁓15 trillion tons of ash and soot were released into the air and the day on Earth became as dark as a moonlit night. As a result of lack of light in plants, photosynthesis slowed down or was suppressed for 1-2 years, which led to a decrease in the concentration of O2 in the atmosphere (for a time while the Earth was closed from sunlight). The temperature on the continents dropped by 28 ° C, in the oceans - by 11 ° C. The disappearance of phytoplankton, the most important element of the food chain in the ocean, has led to the extinction of zooplankton and other marine animals. Depending on the time spent in the stratosphere of sulfate aerosols, the global annual average surface air temperature decreased by 26ºC, and ⁓16 years the temperature was generally below 3ºC. There were conditions reminiscent of the phenomenon of "nuclear winter". This "asteroid winter" has led to a number of negative processes for the life of organisms: reduced food resources and broken food ties. The decrease in temperature affected living conditions, the salt composition of sea and fresh water, the condition of soils, the distribution of nutrients and water on land, and so on.

Given that the introduction of the cosmic body into the Earth's atmosphere affected various aspects of natural conditions, this led to selective extinction. Some organisms, such as terrestrial and aquatic dinosaurs, highly organized planktonic organisms, and a number of others, have failed to tolerate such habitat disturbances; conditions, forms. Subsequently, traces of the "iridium anomaly" in addition to Italy and Denmark were found in other regions in the border layers of the Mesozoic and Cenozoic.

2. Later it turned out that such anomalies exist at the boundary of the Eocene-Oligocene, Permian-Triassic, at the boundary of the Famen-Fran in the Late Devonian and early Phanerozoic. All this indicates that the penetration of cosmic bodies into the Earth's atmosphere in the geological past was not a rare event, but probably occurred with a certain frequency and it is usually associated with the vast majority of large extinctions.

The fall of a large cosmic body should leave traces on the earth's surface in the form of an impact crater. It is established that the oldest of the known craters is located in South Africa. It has a diameter of about 140 km and formed about 2 billion. years ago. Sudbury Crater in Canada was 1.84 ± 0.15 billion. years ago. The vast majority of known impact craters are less than 300 million years old. 65 million years ago, on the border of the Mesozoic and Cenozoic, Karsky, Ust-Karsky, Kamensky and Gusevsky (the last two are located on the Black Sea coast) craters with a diameter of 3 to 25 km appeared. The most likely candidate for large craters formed on the Paleogene Cretaceous is now the Chixulub Crater (Yucatan Peninsula, Mexico). Its age exactly corresponds to this boundary, and numerous features - specific minerals and rocks, geochemical anomalies, etc. - confirm the cosmic origin. In addition, among the rocks in which this crater is located, there are sulfates, which could be the cause of the entry into the atmosphere of a significant amount of sulfur dioxide, which is harmful to living organisms. Data were also obtained on the close, if not identical, age of another large crater - Karsky on Pai-Hoi. There is evidence of the existence of such a crater in the Pacific Ocean. It is assumed that the largest crater from the collapsed asteroid is at the bottom of the Barents Sea. There are other views on the problem of extinction.