Now it just randomly happened to another book on my device (not sideloaded). I haven't even opened the book on the device since I last plugged in.
Trying to right click on the cover and opening it in an image viewer results in nothing happening.
For the non-DRM'd books with weird covers, the covers embedded in the ePub themselves are fine.
I tried to open the cover directory on my Kobo to see if it was a problem with my device but it was all PARSED files.