While this seems quite clearly beyond the spirit of the rule, I’d like to ignore the actual question, and ask a question of my own.
How would a team get the robot to lean over the wall?
If there is a mechanical actuator that causes the robot to lean, that would be expansion, and outside the rules.
If the robot was **designed **to “fall over”, that would be like a 15" tall robot falling on it’s side and driving around the field in a 15" x 20" orientation. Personally, I’d say if a robot is designed to violate the size rules while on a flat surface, it is in violation of the rules.
The robot is meant to be driven in a 13" x 20", but when you raise its arms, the momentum may cause him to loose its balance and fall backwards, hanging against a wall allows it to be able to raise its arms safely without the risk of falling down, but since it leans a bit backwards it may cause it to extend over the 20", not because it is longer than 20", but because its height is inclined backwards.
If the robot is leaning against the side wall, and gains no advantage other than maintaining its balance, then most judges would (and in my opinion should) overlook this. However, if the robot gains a scoring advantage, by leaning against the scoring zone rail and extending itself towards the goal so that it is longer than 20" when the angle of lean is included, then I would expect the robot to be called for a foul.