Seabrook came in hard on the forecheck, lining up Backes for a hit with the Hawks on the power play. However when he came in on the hit the puck was already well gone off the stick of Backes and he was in a dangerous position. The result was a booming hit with the principal point of contact being Seabrook's shoulder right to the face of Backes. There's not much dispute, though; it was a bad hit.
What's interesting, though, is to hear the explanation from the NHL above. There is no talk about the contact to the head in the discipline, just mentions of the hit being examples of interference and charging. No matter what way you get there, though, the result is the same: a bad hit worthy of a three-game suspension.
Backes was severely dazed -- we can all assume what he suffered on the hit -- and had to leave the game. Seabrook, meanwhile, earned a five-minute major and a game misconduct.
The hit from Seabrook already cost the Blackhawks huge in one game as the Blues scored on the man advantage with 6.4 seconds left to force overtime and then won it in the extra period to go up 2-0 in the series. Now it's going to cost them big in the next three games* (*if necessary).
Seabrook played all 82 games this season and had 7 goals with 34 assists while averaging 22:16 per game as one of the Blackhawks' most important defensemen. Clearly it's a pretty massive loss for them. Of course the Blues potentially missing Backes for a few games is massive for them too but it has the Blackhawks in a really precarious spot now heading to Chicago.
Also, while it won't have any supplemental discipline, take note of the video below around the :44 second mark. When Backes is down and struggling to get back up you can hear a Blackhawks player chirp "Wakey wakey, Backes, wakey wakey!"
Not a good very good look there, Hawks.