I'm not to sure but I know if your Princes stay in one place to long they tend to pick up lazy, that is if they didn't have it to start. If you move them around keep them leading battles or leading armies that fight on occasion they don't seem to get these traits.

It also seems to take a really long period of a heir doing nothing to get a lazy trait, but after he gets the first it seems he can get more of the same faster ie slothful.