It happens because the boyfriends are being insecure / distrustful of a man they've never met and/or their own girlfriends, and your friends would rather keep the peace in their relationships by cutting you off.

It sucks, and if it's true that you aren't pining after them or trying to hit on them, then you didn't do anything wrong here. Platonic friendships between men and women are perfectly sane and normal. But to play Devil's Advocate for a second here, I'm sure you're aware that a lot of other men will often pretend to be friends with women while having ulterior motives, so on some level the suspicion is understandable even if unwarranted in your case. Just because you know the truth about your intentions doesn't mean that they do.

I would perhaps try raising the subject when you have the opportunity, like when your fair-weather friend of 10 years breaks up with her boyfriend and you guys have the opportunity to catch up. Don't expect any satisfying answers, though.