There are plenty of unwritten rules in hockey which deal with respect for the opposing team. One of those rules is that you can’t shoot the puck after the horn sounds. Ryan Kesler broke this rule, rifling the puck into Calgary’s empty net as time expired.
The Flames weren’t too happy and jumped Kesler.
The Flames lost 1-0, so it’s fair to assume they weren’t too happy to begin with prior to Kesler’s shot.
Say what you will about hockey and its unwritten rules, but this was a pretty solid scrum after the whistle.