Looks like maybe syncytia...are there any fusogenic agents in your media? have they been infected with virus?

Alternatively, it might be another cell type that has cross-contaminated your culture. Do you have other cell lines growing in parallel?

It could also just be the result of continued mutation of the 293T cells. After all, they are malignant cells that will continue to accumulate genomic changes through time.

What I can see is that your background is not clear which means that the media contains a lot on contaminants (what about the color of your media after one day and two days of culture?). Also it looks like your cells are agglomerating in one point and are not able to divide in a good way to fill more gaps (if I suppose the picture you took here is at least 8 hours after subculture). So my assumptions are, your cells do not have optimal conditions to grow (Media, density or manipulation) plus like previously mentioned maybe these cells accumulated genomic changes (if the number of your cells passage is high).

HEK293 and HEK293-T are what you may consider very sensitive cell lines. They have a very high tendency for clumping(or agglomeration) and if left untended to unfavorable conditions like an acidic medium, will detach from the growth surface and die. Such changes in morphology are very commonly observed. You might observe that they revert to their normal morphology a few hours post a media change. You can avoid this sort of clumping if the cells are evenly distributed in the culture vessel (just give it a shake once you keep it inside the incubator for even distribution). Also, observe this cell line at least twice a day to observe any changes. Hope this helps.