I have no idea what's going on with that, but I do definitely think that garters pick up on hormonal signals from other snakes. I was the person who posted about the shedding; they just sync up way more often than coincidence should allow.

Can you quarantine one of them for a week or two? If they are reacting to the others' signals, that should break the feedback loop. If it's something different, then at least that's out of the question.