Smilin_Ed, Huck,
Have you read the report?
If not I can try and pull out the relevant bits and translate them.
In brief, they had been running all four engines for about three minutes already, waiting for the oil to get warm enough to check for a leak.
So by that time their hands would have come off the throttles, thinking the parking brake was holding the aircraft adequately.
In reality, the engine thrust equated almost exactly the holding power of the brakes for those three minutes, and then (possibly because the fuel burned, hence a/c becoming lighter) overcame the brakes.
CJ