Not BPs, but my garters do it. I think it's got to be some sort of hormonal thing that affects nearby snakes or something because it happens way more than you'd think to just be coincidence.
I have 18 for example, and when one starts shedding normally 3-4 will go within the next week or so.
I don't believe its a hormonal thing, its more of a: "I keep all my snakes on the same feeding schedule, so they are all growing at the same rate" thing.
Very interesting. Perhaps a gross comparison but in a funny way it reminds me of menstruation. Happens to everyone in the house at around the same time and they all get crabby.