What triggers the auto-upgrading campfire glitch?

I've seen it happen a couple of times now. My own tapping pattern is something like:

- I tap mostly zombies
- I will get an upgraded hedge every so often.
- When the next upgrade level nets a tent (skeletons), I switch to tapping those.
- Then I switch back to zombies. Zombies come after skeletons, so at that point, I quickly level-up again

At this point, it takes a couple of days, but I eventually get a leveled up hedge again. It appears to skip the spooky shed and goes right to the next hedge.