A toddler who is potty trained and then starts peeing his pants again could be caused by potty training regression.
Some toddlers will go to the toilet on their own without any issue and will pee and poop in the potty which is good and then all of a sudden they may start peeing their pants again.
Some toddlers start peeing their pants again on purpose to try to get the attention of the parents again especially if you have had another baby and the toddler sees you changing their diapers.
The toddler may miss the attention they were getting from you so they start regressing and wanting you to put them back in diapers.
My 8 year old son started doing that for some reason and would pee his pants on purpose and so I told him that if he did it again he would be wearing diapers.
So he did pee his pants again and I put him back in diapers for awhile and he seemed to actually love it.
Eventually after about 6 months of me making him wear diapers and changing his diapers he decided to start using the toilet.
I think it was a phase and he just grew out of it.
Toddlers do the same thing sometimes.
Or your toddler may just be too busy to go to the toilet and sometimes you have to remind them to go to the toilet.
If your toddler keeps peeing his pants tell him that he has to go back to diapers until he can keep his diaper dry.
Sometimes that is enough of a motivator to get them to go to the toilet.