have you accidentally fart while someone rim your anus or while someone insert their penis inside your anus ?
No, not when rimming or putting it in, but upon taking it out, yes. Both when I was the bottom and when I was the top. I think part of it is due to ordinary air getting pumped in during the... ah... pumping, and from the anal sphincter being stretched loose for a while.
If you're gonna do anal you just gotta take such things in stride, and understand that a bottom will sometimes do that. It's not really his fault, so don't make it more awkward for him with rude, angry or insulting remarks.