Top layer of kimchi turning brown

I made some (pre-chopped) kimchi a couple of days ago (Sunday), and left it out to ferment at room temperature in Toronto, Ontario, Canada (it’s been between 10-20 °C here for the past few days).

Yesterday (Tuesday) I checked the containers. Because of all of the air that had been released, the containers were almost overflowing, so I took some kimchi out from each container, pushed everything down, closed the containers, and then left them to ferment a while longer.

This morning when I got up, I noticed that in the top layer of kimchi, the red pepper flakes had all turned brown. I’m guessing the red pepper flakes oxidized due to exposure to the air … but I’ve never seen that happen before in all of the other times I’ve made kimchi.

Has anybody had this happen to them or have any insight? Thanks!