Toddlers pee their pants after potty training as their bladder is still weak and they still are not completely used to holding it to go to the toilet.
After all the toddler was used to wearing using a diaper and going pee anytime they needed too so it can still take some time for the toddlers bladder to mature and strengthen and for them to get used to holding their pee.
Also some toddlers may regress in potty training due to psychological stress or seeking attention such as when a new baby is around etc.
Most often though the wetting of the pants in toddlers and children after potty training is not deliberate.
A toddler should go to the bathroom every 30 to 60 minutes when potty training for the first couple of days and then if that goes well they can go longer between going to the potty.
When potty training a toddler the toddler should sit on the toilet for up to 5 minutes.
If the child has not went pee or poop in 5 minutes then they should be allowed off the toilet and not forced to sit on it any longer.
Then when you think the toddler is ready to pee or poop again you take then to the toilet and sit them on the potty for another 5 minutes.
Never get upset or punish your toddler when potty training as it will make it worse.
Sometimes a toddler will have a bowel movement or pee again once the diaper or pull up is back on which is normal because the diaper feels normal them.
Pull Ups do sometimes confuse potty training toddlers.
However some toddlers potty train just fine when wearing pull ups.
Pull Ups most often hinder potty training and confuse the child as a pull up feels like a diaper because it is.
Potty training a child is better using underwear so they feel wet.
For some toddlers pull ups can hinder potty training while for others pull ups can help potty training.
Pull Ups can feel like diapers because in reality pull ups are just pull on style diapers although they are less absorbent.
If a toddler is wearing pull ups and pees in them then they won't feel wet like they would when wearing underwear.
Just like a diaper the pull up absorbs the pee and so the toddler does not feel wet.