Team stalling at enemy base
It took 4 minutes to take down the first Nexus tower, then 6 minutes to kill the second one, and then 3 minutes to kill the Nexus. They were 21-2, so it's not like they were weak and they picked up another 59 kills while stalling.
I watched a few replays and they usually pushed down these 3 objectives in seconds. I don't know if it's okay to post replays, I can post it later if it is.