My DD used to love to stand/squat to do a poo in her diaper also, this was for several months after we got the peeing accomplished. She would go get a diaper and tell me she had to go. I also kept her in diapers to sleep in at night until she was at least 3, until she woke up in them dry for two weeks. I found the pull-ups to be too expensive, as she was never a good sleeper, I certainly didn't want her worrying about peeing in the night and having to get up out of bed.
Anyway, we never really got down to why she didn't want to poo in the pottyI don't know if it was the sound it made or what. I just allowed her to poo in the diaper, telling her what a good job she was doing, then asking if she wanted to try it on the potty like Mommie doeseventually she did, it did take her a few months though!