There was a big discussion about this happening on the garter snake forum I'm on. People would consistently have snakes of different ages/sizes shedding at the same time. I've noticed with my collection, that (with the exception of one male who is twice the expected size for a male and twice the expected lifespan for garters... so he's kind of my outlier) my smaller males will go into shed right around the same time as my near-adult female, or the older large male. But I've never seen the female or the larger male go into shed right after one of the smaller males.
A lot of people on the garter forums had noticed similar things happening in their collections. It was enough to make me wonder if it was related to hormonal triggers or something.

Of course I imagine garters would be more susceptible to hormonal signals than BPs just because they tend to hang out in large groups in the wild.