It was biting with its feet

The T-rex could already hunt you. It just bit with its feet.

The first strike system measured reach from origin to origin, which for a voxel animal is the ground under it. Standing in front of the mouth did not matter. Standing on its toes did. And the mouth never opened. The jaw was scenery on a body that already had an attack pose.

The bite now lives on the jaw. At the moment of impact the hit is tested against that part of the rig, not the model centre, so the dangerous end is actually the face. The animation is a two-phase snap: the mouth opens through the windup, then slams shut on the tick that does the damage. Range and standoff are per species, so a T-rex has to get its head on you rather than tagging you with its ankles from a body-length away. While it hunts it can look at what it is chasing, which sounds small until you have watched one bite the floor.

Other animals already strike with different parts. A tail swing is a different problem. For a predator that is supposed to bite, the mouth had to do the work.