dgutte - one minor correction, I think - you will not be 'pushed into your seat' UNTIL the brakes come off!
My guesses:
Normally, in winter where engine icing is more prevalent, this is a normal procedure. The engine is run up (737) to 70% of max RPM (NOT 'full power') for about 15 seconds to dispel any ice in the engine. This is required even in summer if the temperature is below 10C (EGNT
) and there is, say, rain.
Now, on odd days (like Friday 13th
) pilots SOMETIMES feel a LITTLE uneasy about runway length etc and MAY run up to around 70% before letting the brakes off. This obviously (as you said) gives you improved performance.
Surprisingly, for a flight to Iceland from EGNT, the runway MAY be a little limiting, due to either engine anti-ice requirements, low pressure, adverse winds or just a heavy aircraft. The crew MAY have felt easier by doing this run-up on that occasion.
ZTH - I guess no icing problem
, but performance can be an issue expecially if you are going ZTH-NCL. The crew may, again, have felt easier with this.
Finally, it could have been some obscure technical procedure.