I am not sure that this is abnormal. Many people think you just sit there long enough for whatever is at the exit to "explode" out, and you are done. But, unlike a rectum that contracts when full, the j-pouch empties by gravity. So, yeah, it is not atypical for more stool to drop down after the pouch empties. You clean up, stand up to wash your hands, and "whoops" there's more that wants out. The longer it has been since you last emptied, the more there is to come out.
Sometimes what I've eaten has an impact too.
So, I've just learned to be more patient.
But, if this is causing you pain, then be sure to bring it to your doctor's attention.
Jan