I'm not a hockey ref but I can give you an answer since I saw this situation happen a couple of times in the NHL.
It actually makes sense if you take the time to think about it: the offender commited a tripping penalty and the offended player exagerated his fall, hoping to get a power play for his team, but not knowing that the ref would have called a penalty anyway. So the ref penalized the trip, but also the unsportsmanlike acting.
Just my 2 cents, but I could bet I'm right!
