Cornered

One thing people who believe that Tyrannosaurus Rex was purely a scavenger tend to forget is that most of it's prey choices are actually slower than T-Rex is.

That being said, many of these animals also tend to be armed with formidable weapons and/or heavy armor.

Given that T-Rex is the only really large carnivore in it's habitat, and the only one with the bite power strong enough to possibly pierce this thick armor. Why else would they need it if T-rex isn't the threat.