Spoilersssss.......!!!!!!
My best guess is Cap nudged the gauntlet out of his hand and Thanos was astonished at how he was able to do it.
My reasoning for this guess? It seems to echo a similar beat in Avengers: Age of Ultron when during the party scene, Captain America was the only one who came close to shifting Thor’s hammer Mjolnir when others miserably failed. He tilted it just a bit but it was enough to give Thor a scare and send a frown on his face.
Thanos has a similar moment. That an undeterred and unafraid Captain America alone is able to make some progress in removing the Gauntlet out of Thanos’ hand unsettles even the Mad Titan Thanos himself.
