I seem to remember as well that the coffee was reported as instantly boiling when sugar was added.

If that's the case (and no reason to disbelieve the plaintiff), that's a superheated liquid. In other words, it is a liquid above its own boiling point and waiting for a cavitation event (something for bubbles to hold on to) to instantly boil. And this seems to coincide with the coffee explosion and burns from the plantiff.

Mythbusters video of an example of superheated water: https://www.youtube.com/watch?v=1_OXM4mr_i0