Here's my SFO-specific guess: You were landing on runway 28-Left. Another plane had just landed on 28-Right. When planes taxi from 28R to the terminal, they have to cross 28L. Standard procedure is to turn off 28R and hold short of 28L to wait for landing traffic. A possible explanation is the 28R plane stopped but didn't quite hold short, was sticking into your runway, and couldn't clear fast enough from a stop.

Just a guess, tho. There's usually plenty of separation to deal with these things. Would be interesting if you found the liveatc audio and let us know.

(It should be reassuring to know these jets can take off from a landing-in-progress with no real difficulty...)