Stress? Well.....Smith was up for a big professional award. Rock made a joke, the taste and intent of which can be debated, but even assuming for the sake of argument that his intentions were the worst (and the tape indicates that Smith was laughing at the joke before he looked at his wife and realized Pinkett Smith was Not Amused), what followed was pretty egregious. Jeez, he strode up, sucker-smacked the guy, and then scooted back to his seat to launch F-bombs. He was not escorted from the building. He was allowed to accept his award, talk about his special problems as a big star, got a bizarre standing O, and then went on to boogie all night as if nothing had happened.
Hardly the crime of the century, but not small potatoes, and the Academy is acting accordingly, if belatedly. As for SAG, I imagine this comes under the category of a workplace incident and certain processes follow from that. I doubt Smith will be expelled, there are other lesser actions they can take.