The fiery Canadiens forward has not missed any games this season due to injuries, but he was suspended for 5 games for a hit to the head of one of Pageau's teammates earlier this season.
I don't know if that explains this fight, but we know that players have long memories in the NHL and that offenders are invited to pay their debts.
In any case, Pageau was an opponent of Gallagher's size, and it got rough enough. We can even give the victory to the Canadiens' number 11.