You asked for some speculation -- Which may not be correct, but in keeping with your request....

I think that the observation that the deformation is bilateral and symetrical may be instructive. Not having picture makes is tough to speculate a lot.

Do you have a mast base that extends some distance up the mast? (Say 4 or 5"?)

Some racing boats and others have a "positive" restraint which keeps the mast from "falling-out" if you lose the top or the stays in a roll-over or bad knock down.

it's possible that the damage occured during re-rigging the spar before the stays were in at some point. I doubt its the function of torsion or some other twisting from your description. I've heard of spars which were "dropped a bit" on the parking lot after they'd been pulled? Is there any sign on the bottom edge of the spar of some impact.

As D'Arcy says, pictures might be informative here!