Did he only do shrugs? He did nothing that worked his shoulders but shrugs? If not, then how does he know it was the shrugs? There are way too many factors for this guy to just say it was a particular exercise. I'll give the schmuck the benefit of the doubt and say shrugs did him in. But maybe it wasn't the exercise in particular, perhaps he let his ego choose the weight for him. Maybe he did them with bad form.
Hell, he just may have had genetically weak shoulder joints.
Well, you get the point...or at least I hope you do.