If weather conditions are right for fan blade icing, then the pilot will sometimes spool up and check instruments for excessive vibration before he begins his roll. If there are vibrations present, as on a recent CO
flight I took out of ANC
on a 737-900, you will hear the engines spool up and down, in an effort to sling the ice off of the blades.
If you listen to ATC at a wintry airport, you'll often hear the heavies request thirty seconds for engine run up.
Not saying that this is what was going on for your flight, but it's a likely scenario.
I've also seen pilots do this in the dead of a hot, dry summer... for which (as a mechanic) I have no explanation.