Of the ball. I think he wasn’t sure if the ball would drop or not and he stayed off the base for a while, then he realized late that it was going to be caught so he hastily went to tag up and either 1) took his eye off the ball/fielder briefly while he was tagging then guessed on when it was caught or 2) could have lost the ball in the sun and thought the defender caught it when he hadn’t yet. The fact that he went back to the bag at all confirms he knows the rule but I think he legitimately lost track of the ball but then “guessed” at when he caught it and then when he was called out, he questioned the result because he was either embarrassed he lost track of the ball in a high pressure situation or he legitimately thought he saw him catch it when he didn’t.