S.W.
My middle child went through much the same. I never saw it as regression, though. To me it was simply like you were saying...she just was waiting too long to go. I didn't do anything to try to change it, just matter-of-factly had her change her own panties and pants (or whatever was wet) and put the wet ones in the laundry. I think she got sick of all the trouble she had to go through and just eventually stopped waiting so long. The stage passed.