Hi G-Style,
Is it from anal stimulation or anal sex that your partner is cumming anally? Or during regular intercourse she is cumming from the anus?
I've done some research and it appears that it might be a possibility but most people put it down to that the vagina can get soooo wet that it appears she may be cumming anally as it can drip down from the vagina to the anus to appear as she may have cum from her anus. It would be rare that she would/could actually come from the anus as it is very rare.
Some people argue that it is g-spot stimulation but it would be pretty tough to do as the g-spot is nowhere near the anus. I would believe that maybe she is just more turned on and she has gushed/squirted and has extra vaginal fluids that's appearing like it comes from the anus. Also, it could be a learned behavior as she is turned on with any sort of sexual play and foreplay.
Hope this helps some and happy playing!