This is not a new procedure. The first time it happened to me was at least 3 or 4 years ago. I thought that this was a targeted flight (LHR/IAD) and that they didn't want anyone to have the opportunity to get back on the plane to abandon the contraband or somehow otherwise run off the plane through an emergency exit. I was in Row 1 on a 767 so was off the plane right away - IMMEDIATELY the agents started shouting "single file" and the dogs went at it. I don't know if they stopped anyone.