Unclear cause

See also: blank protagonist

This is generally caused by lack of written or spoken story elements and relevant background info on the protagonist, or when they're present, don't contribute to actually telling the player why would the protagonist do any of the stuff.

For example, a soldier suddenly deciding to battle through their allied forces to kill his/her own leader. A police going on a killing spree (especially of non-criminals). A couch potato computer nerd fighting to join the Olympics. And so forth.

Sandbox games do not belong here, nor games with no clear goal, nor games that by their type define the reasons (e.g. exploration).