I think it was the Bill's scheme. They came out with some formations that the Pats didn't expect and I think their reads were messed up. They looked a little confused by it. They had it figured out by the second series though.
That's usually why teams tend to get into their base defense until they see what the other team is doing. The Bills did to us what we did to the Chargers in terms of coming out in an unexpected way. Fortunately, we are better at adjusting to it.