The NHL gave teams five games to decide what to do with youthful players who are still young enough to play for junior teams. In Hamilton's case, it was time for him to graduate from the OHL Niagara IceDogs and join the big time. He had already played four Bruins games prior to Jan. 28, too.
Hamilton has already achieved quite a lot in his career to date, including winning the OHL's Max Kaminsky Trophy, awarded to the league's most outstanding defenseman, in 2012. He was also on the bronze-winning Canadian team at the 2012 World Junior Championship.
He'd basically outgrown junior hockey, anyway--when he won the Kaminsky Trophy, he scored 72 points, then had 41 points in a 33-game return to the IceDogs necessitated only by the NHL lockout. Now he gets to prove himself in the big time.