This is a bug I couldn't reproduce.
I had no talent points allocated, I built a horadric cube on the first spot from the bottom, it had a staff, a sword and a pumpkin. Then all of the sudden i see the rounds spawn one after another without seeing any monster.
Turns out the cube had a great 0s attackspeed so was going insta-machinegun on the poor monster that were obliterated as soon as they got in range. i could have won the game but i don't like exploits so i gave it a potion of speed and it became normal again.
As first said, i tried to recreate the conditions but couldn't get it to happen again.