Having problems with constipation can cause accidents (and you might not even realize they have constipation). My niece was having problems with accidents for a LONG time (she's 7) and my sis thought it was stubbornness but it turned out that she was(is- they're working in it) severely constipated.
Also kids go through periods of regression as they learn new skills. YDS would be dry for a couple of weeks and then have a week where he would not make it to the potty all week. I'm hoping he's done with that now that he's in undies full time but...
I liked having him in trainers that held as much as diapers when he was having wet periods and then less absorbency when he was in a dry period. Much less stress.