I have no idea what caused this; After entering Matchmaking right after a game, this is what the screen displayed:

Apparently, the timer kept counting during the game I just came from (the time approximately matches). I think I was on the Learn tab when I returned to the menu, but I don't quite remember. Regardless, something for some unknown reason caused that timer to not stop; I have no idea how to reproduce it, as it had never happened before and it's never happened since.