It always used to be the other way round, that those in porn would pull out to cum when no-one actually having sex would do that. At least that is/was the case with straight porn. I had always assumed the issue in straight porn was that, while men loved to be pushing in as far as they can and unloading inside the woman, that being the most natural and instinctive way to cum, there wasn't much for someone watching to see. Sex is really meant to be done more than watched, yet porn is visual medium.
So, external ejaculation became the norm in both straight and gay porn. Perhaps some producers now want to do something different to have a unique selling point? Of course, any fashion can be copied.