This seems to happen randomly and I am unable to reliable cause it to happen.

I use multiple different sets to control which trinket queues I am using during a fight. Sometimes when the queue is switching my trinkets, and I feign death to let it happen the active set is also switched. This messes up the remainder of the queue because the set that gets activated has a different queue priority.

I recorded a video during raid last night: https://www.twitch.tv/videos/805235760
my currently equipped trinkets and the current active set are on the lower left side of the screen.

I did double check to make sure there are no hotkeys bound to changing my active set.

This has been bothering me a lot and tried a bunch of things to fix it. What makes it worse is it seems to only happen in raids. I even tried being an instance and in combat but could not reproduce it (by going to DireMaul North and attacking the spirits at the end of the instance)

Last night for raid I turned off ItemRack events and it never happened to me. I am reasonably convinced there is some logic that triggers the set change on events that gets confused in some cases.
It may be worth noting, I normally only have mount event enabled, but with the setting to not change sets while in raids.