Have you experienced it? And how did it happen?

I have experienced it by colleagues but it doesn't affect me, simply because I ignore these people. Also, I wonder if international students happen to receive more academic bulling than native students.

I think it depends on the make-up of the research student group in the department. Sometimes it's supportive, other times there are just too may egotistical types who view everyone else as competition to be stamped all over. My PhD years were blighted by a few obnoxious people, who were convinced they were god's gift to the subject, spent their entire time telling everyone how much better they were than the other PhD students and all the academic staff (they were really horrible about the early career academics in the department) and that anyone not using the same approach as them were losers. Interestingly none of them got academic jobs, so maybe karma kicked in. Mind you, I imagine it's exactly the same in any other sector that attracts driven and ambitious people. Certainly I've heard that law is like that. Most of the international students I studied with were not aiming for UK academic jobs (on the whole the terms of their funding meant that they had to return home and take a university job there), and so they were protected a bit from this. Where the international students really suffered was undergraduates complaining about their teaching because they had accents (but spoke perfectly good English and knew their stuff).

It's not just PhD students. I've observed this among professors as well.

Egos .... putting others down who don't agree with them! Not helping their students who may have different ideas to them. Just being obnoxious! Not promoting discussion because their view is correct! You get the idea ... it's all about them. Of course the method they use depends on their personality and it can be unprofessional and not what one expects of a professor!!! They can be rather forthright or aggressive about their views!