1) Lots of Mac and Linux fanboys like to mindlessly bash microsoft like... all the time to justify their purchases and therefore everything microsoft makes, be it computer, videogamesystem or toothbrush sucks.
2) It's cutting into PS3 territory and was threating it for a while... pissing off a lot of PS2 fanboys who thought the PS3 was going to be dominant, and it's easier to pick on the 360 then the Wii which is kicking way too much ass. (Though some setill do.)
3) The original Xbox pissed off Gamecube fans because it came in that generation and did as well as the Gamecube while both got their asses handed to them by the PS2.... which most gamecube fans couldn't pick on because it was too dominant... so they transfered their hate to the 360... (and also take the time to kick the playstation brand while it's down.)
My theories anyway.
