If you've replaced the throttle cables and it now works as it should - I would say that is your answer.
That would have been my first guess. Even when cables seem OK, the throttle won't return, and just by fitting new ones, the throttle snaps shut.
If it works ok now, I would say you've cracked it.
Just check it and lube the cables regularly, and you know what to do if it happens again.
