Looking at the twitter video, the ref is on the scene within 2 seconds at most, from the left side.
This would suggest that he was 5-10 yards away, with a clear side-on view of what happened and he correctly deemed it a yellow card.
How then, can the ref possibly believe that the 4th Official has seen the incident better than him, from 50 yards away, when his view of Jack is partly obscured by Broadfoot?
If he saw the same video that is on twitter, then it isn't a red card. Broadfoot is repeatedly trying to stand on Jack's feet. Jack pulls his feet away, pushes Broadfoot and moves towards him. Broadfoot then takes a dive, holding his face. On the basis of that video, there is no headbutt and no kick. It is a yellow card for the push, no more.
The BBC Live Text does not mention Jack being booked earlier in the game, so that rules out the possibility of the ref forgetting that he had booked Jack already.... unless they missed it.
Once again, it looks like an Official has wrongly acted upon the reaction of an opposition player, rather than the actions of a Rangers player, which is becoming a common occurrence.