How did Bobby end up in the wheelchair? I missed a few episodes.

He was possessed by a demon and "Meg" told him to kill Dean but Bobby reacted the best way he could and stabbed himself :(

Another reason of Bobby's paralysis is because when Zachariah was threatening Dean to accept Michael, he was hurting him, Sam, and Bobby. When Castiel popped up, he told Zachariah to "fix these boys", but Bobby was never "fixed"