Just a PP so dont shoot me!
Is at all possible they had a problem with brakes not fully releasing?
Alloys dont normally emit too many sparks, but brakes certainly can.
Are there any brake temperature indicators on the flight deck?
My first thoughts were that it had to be weight related, but after many learned observations, perhaps not, although theft is of course an art form, and I am sure a determined team could find away around the checks and balances(no pun intended).