To be fair, anal is massively popular in non-BDSM porn too. It is really mainstream. And I don't think it has come from BDSM. It has been popular for years.
I hate the normalising of anal sex in porn, it makes it the norm rather than a fringe activity.
The difference between the normalising of anal sex in porn, and BDSM in porn, is that BDSM cannot be accurately portrayed with its emphasis on dynamics.
So whilst anal sex is always anal sex, BDSM porn is most often just violence. I condemn it wholeheartedly, as has everyone here who is into BDSM.