I think he lost his grip on the ball briefly as he loaded up to throw to first. He quickly brought the ball to the glove as if to adjust the grip, but it was too late.
THe angle shown didn't show the progress of the runner so it's remotely possible he held his initial throw due to the slowly hit grounder, but Cust isn't fast, so I think he just lost the grip briefly.
There you go mindfacting again.
