It's more common than you'd think, especially for park riders. If you land on your butt hard enough you can trigger a spasm.
I've had the spasm happen twice and I had to clench insanely hard both times to avoid a very messy walk home. Never had anything come out, but pretty dam close.
Quoted for truth. Last year early season, I hit a jump at Stevens pass called the waterfall. Over shot the landing a bit to much and landed hard on my ass bone. Thought I shit myself and went strait to the bathroom clenching. Didn't poop myself but I thought I did. Damn painful and it ended my day.