Beats me, but as Senseamp mentioned, that's exactly when the release can do the most collateral damage to the GOP legislators running for election/re-election.
Grudge? I see it more as Trump, in all of his arrogance and ignorance, diverting attention away from himself without pausing to fully understand the consequences of that act. I really don't believe he considered how this accusation of his can backfire in his face with "stupendous" results that may actually help him emerge out of that "stupor" he's been in since being elected.