It's actually a late sign of his growing daily frustration with his metastasizing Alzheimer's, as he loses his grip more and more on what one would loosely refer to as "his mind". (I'm being rhetorically generous here.)

He had shit for brains on his best days at the height of his powers, and the decline now is so precipitous that one day, very soon, someone's going to literally be forced to get him a coat with sleeves that fasten in the back, have him declared unfit to change his own underpants, let alone run a country, and the Democrats' second-worst fear will be realized: Acting President Kneepads.