West, an interloper at past award events, got up there and took the mike from Swift just after she started thanking the crowd. West said Beyonce had one of the best videos going. He gave the mike back to Swift, who was shook up by West and could not finish.
West was later given the boot from the show as he should have been. Not quite sure what he thought he was doing, but he comes off as a first class jerk in pulling off such a stunt. He also obviously embarrassed Beyonce, who later asked Swift to return to the stage. Swift probably has enough sense to release that Beyonce, who seems to have a lot of class, would never do something like this. She doesn't need to.
Not sure why West feels compelled to do so. His action not only made him look bad – he drew a ton of well-deserved boos, but Beyonce as well. As for Swift, she'll be getting a ton of sympathy, something she wasn't exactly looking for at the outset of the night.