Doors have been beaten up looks like to me. Not handled well for the most part. That and possible poor quality doors.

Humidity swings, bad paint job, and low quality doors.

All of the above, along with being trimmed on the bottom, and not a very sharp blade.

^^^And the bottoms probably were not primed & painted after trimming.

What you have is veneer splitting caused by incompetent trimming, possibly with a dull plane.