Upon taunting/rage with heavy sauce and, to a lesser extent Gaben, your hitbox enlarges or something, causing you to get stuck in the floor on slopes, or allow pyros to push you into scenery, getting you permanently stuck. Spies and ubered players also can get stuck inside of you, making you unable to move also. If any of these things happen it means you have to lose and have wasted your queue points trying to play a fun boss. Thanks.