Normally, the causes of boredom can be due to the brain's own mechanisms, and circumstances from outside. About these reasons, wrote brilliant writers - Dickens, Goncharov (Oblomov), scientists, Nobel laureates (Ivan Pavlov) ... And scientists and writers took into account the genetic characteristics of the individual, the features of human education, the features of the formation of the worldview, goals and incentives in life. I'm sure that in extreme situations that threaten the life of a person, a person with normal psyche will never experience boredom.

Please, also have a look in the Attachment.

It seems to me that boredom is the opposite of flow for the brain. It doesn't really have a specific mechanism but likely arises from the attentional circuits when a person is asked to devote their attention to something that they are do not find interesting or stimulating or when the environment lacks any stimulus that attention finds stimulating and so is in a continuous search loop.

This article describes boredom in terms of attention. No room to summarize here but very comprehensive article including extensive theoretical description along with explanation of brain mechanisms.

Article The Unengaged Mind: Defining Boredom in Terms of Attention

This article contextualizes these mechanisms in the default mode network.

https://link.springer.com/article/10.1007/s00221-016-4617-5

This article on the positives of boredom summarizes the above findings more succinctly.

https://philarchive.org/archive/ELPTGO

This article describes brain mechanisms of boredom as part of operant conditioning.

Chapter Curiosity and Boredom Based on Prediction Error as Novel Int...