Because Thor was on the verge of starting an international crisis. Slokovia, a small Balkan nation near Latveria, had this fascistic government that was oppressing its people. Some of its people prayed to Thor, since Thor's increased involvement on Earth had gained him a massive worldwide religious following dubbed the Church of Thor. Thor heard their prayers and decided to intervene. Iron Man tried to convince Thor that the issues were a lot more complicated and that involving himself could be catastrophic. Thor brushed him off. Iron Man gave him an ultimatum: remove himself and the Asgardians from Slokovia or Iron Man would remove him. Thor brushed Iron Man off again.
Earlier, Thor had given Stark/Fujikawa a jewel empowered by the Odinpower for study. He hoped Tony's scientists could find some way to duplicate the energy source and mass-produce it. Ever looking to improve his armor, Tony built a suit of armor that was basically a reactor for harnessing the jewel's energy. He suited up and went back to Slokovia and delivered Thor a final warning. Thor, insulted that Tony would betray their friendship by turning a gift into a weapon against him, got really pissed. They fought, Dr. Doom escalated matters by remote-controlling Slokovia's weapons (sold to them by Doom, of course) and attacking the Asgardians, the Asgardians retaliated, the UN sent in ground troops, and the whole powder keg basically exploded.
Captain America showed up and stalled the UN's forces from going in, claiming he could talk Thor down. By this time Thor had demolished Iron Man's Thorbuster armor and was about to go to work on Iron Man himself (he had another suit of armor on underneath the Thorbuster armor). Cap tried to talk Thor down but Thor, in a rage, spun and attacked Cap blindly. He hit Cap's shield and dented it (which is what the pic Anubis posted shows). Horrified that he hit Cap, one of his closest friends, with a blow that, if not for his shield, would've pretty much vaporized him, Thor realized that he was going off the deep end a bit. Rather than coming all the way back from the edge, though, he just quit the Avengers and pulled Asgard's forces out of Slokovia.