I have been in a situation where everyone gets Pre-Check when the K-9 unit is working.
I'm guessing this is what happened, although I would have thought the OP would have mentioned a dog if there was one...
When they are using a K-9 then all passengers get the equivalent of Pre-check (shoes left on, electronics normally left in, metal detector used, etc).
I've hit this occasionally at SFO, and oddly enough more frequently at SJC (possibly due to there only being 2 screening points?). Definitely makes the whole process much quicker, although going through SJC last week I managed to hit the line right as they had sent a 'decoy' past the dog, and he had correctly detected it, so we had to wait in line for a few minutes whilst the handler played a game with the dog as it's "reward".