I have heard this reported as a transient symptom, while under the influence of MDMA or shortly thereafter, in subjects who have used it recreationally. I have heard it said that MDMA is sometiems cut with strychnine (just why I would wonder) or somehow deteriorates into strychnine. Perhaps MDMA itself causes neck and jaw muscle rigidity.
Anyone who takes a drug prepared or sold illegally is very foolish. It could be anything, and the people who prepare it are not accountable and are probably irresponsible.
Jaw clenching and grinding teeth were the reason behind so many "club kids" sporting pacifiers at rave parties, or at least it was back in the day when I was going to such places. I'd think nine years straight of this would very likely lead to painful rigidity around the neck and jaw.