Inventory functioning in shift-click mode

I was playing pvp and I went to get something out of my inventory, and even though I wasn't pressing shift, it acted as a shift-click.

What I expected to happen:
To pick up the item and move it too my hotbar.

What actually happened:
I had to go to my hotbar, clear a space, then go back and click it.

I'm not sure how to reproduce, as it happened randomly. I exited the game and it seemed to stay if I entered a different server or entered a single-player game.