Where Fluttershy loves peeing her wetsuit, Pinkie wets her snow pants often. Sometimes she’d just have too much fun playing in the snow, and she learned pretty quickly that snow pants wettings are easily concealed, so usually no one has any idea. And she can see why Fluttershy likes peeing her wetsuit so much. Here she forgot to use the bathroom before trying to get the souffle to Rarity, so in the middle of the big snowball fight, she just lets it out in her snow pants where she is, not wanting her desperation to be a distraction from her mission. She’s not sure whether Sunset can hear her peeing, but if she can, she’s pretending otherwise.
 
Spoilers for ending of segment: Rarity is oblivious to how wet Pinkie is when she reaches her. And of course, Pinkie is pretty sure Rainbow Dash did the same that day, after spending two days as a snowman. She finds this amusing, and considers telling her she does it all the time. (As of canon, Rule of Funny was probably in effect there keeping her dry but it’s fun to fantasize)
 
Don’t expect as many of these as with Fluttershy, I wanted to do this once, for something similar but a little different. This is probably the only Pinkie snow pants wetting I’ll do.