They haven't given a reason for attacking someone in a long while anyway. Just the usual "We're the Hounds Of Justice, believe in The Shield, blah blah blah" shtick.
The Hounds Of Justice is just a nickname at this point; "justice" isn't really their gimmick and arguably hasn't been since it was revealed that Paul Heyman hired them to help CM Punk retain the WWE Title (shame they didn't build more on that, I guess.) They've already long transformed into being just another dominant heel group at this point, complete with wanting to win all the gold and everything. I'm personally fine with it because we all assumed that that would eventually be revealed to be their true motive anyway, three guys coming in from NXT wanting to make the biggest impact possible by going after anyone and everyone they could.
So, I look at them attacking Mark Henry as just them being fierce and flaunting their prowess as a group again. Plus, it probably gives Mark some time off and gives him a reason to turn face since I'm guessing he's getting ready to actually retire and wants to go out as a fan favorite perhaps.