In Mark's defence: if a security person is standing and already in a state of readiness then he is less likely to use his arms forcefully (but his body, propped up by his leg muscles) to stop her, than someone who has been sitting for quite a while and is not physiologically prepared for action. Mark's leg muscles not being watmed up, he wasn't in a state where he could block her, he used his arms to do the work, hence the appearance of excessive force.
Probably he shouldn't have done anything