There are people who do this for fun and they’re probably less weird than her, and she finds farts to be funny, so it wouldn’t be anywhere close to a stretch to imagine that she loves wetting herself when no one’s around. (I only say “when no one’s around” because of how desperately she tries (and succeeds, despite being hit in the head by an opening door) to hold it in The Last Roundup) Or someone like Rainbow Dash or Fluttershy even, but if anyone does this it’s most likely Pinkie.