Good God, you mean we actually got the Slug away on time?
Until recently, we didn't even have the crew at the aircraft until departure time.
I would have suggested it was simply the captain showing his frustration at yet another delay, and since no-one can see him shaking his fist from the threshold, he gives us a good blast with his noisy old engines instead.
Seriously though, he could quite easily be performance limited for a reduced power take off as the a/c is generally full (approx 170 pax or so with similar quantities of hold luggage) and with a fairly hefty fuel load (about 14t I think).