I think it's a combo of lazy writing and lazy editing - because the "I Want You Back" scene was cut, we have no idea what happened at Dalton to make everyone so aggressive. Blaine was clearly pissed at the Warblers, and Sebastian seemed more aggressive than usual too. I feel like seeing that whole sequence of events would explain how the Warblers could go from Blaine Anderson fanboys to participating in a slushie fest on Blaine's boyfriend, WHO USED TO BE THEIR FRIEND TOO. Because Blaine wasn't the intended target. *sigh*