Whenever I mage my girlfriend's bum cheeks it makes her fart?

My gf has a really lovely, y bum so during I love to have her lying on her stomach naked while I lovingly and pionately mage her ocks - however it nearly always causes her to fart. She usually giggles and I laugh it off too, but sometimes it gets a little bit annoying and offputting, and kind of ruins the mood slightly (and sometimes it's kinda stinky). Why does she fart when I rub her ? And is there a way we could avoid this happening?