they started overturning the 2 unused nest boxes.... they dumped the 2 unused boxes that night....they're overturning the nest box that the majority of them use!
I'm guessing they try to sit on the edge of the nestbox, and that makes it tip over.
Chickens like to sit on things in the daytime, which could explain the daytime tipping. Chickens especially like to sleep on top of things at night, usually with some squabblings as they settle who gets to sleep where. So some of them might be trying to sleep on top of the nestboxes, or use nestboxes as a step on the way up to they do sleep, which would explain the nighttime tipping as well.
They started pulling the fake eggs out and "playing soccer" with them!... I out fresh bedding in all the boxes, they dumped the 2 unused boxes that night.
If they were scratching the bedding out of the nestboxes, they might have been trying to scratch around and forage for food. Even when they have plenty of food in the feeder, chickens really like to scratch around and look for new bits to eat. They may also have been trying to see if those fake eggs were good to eat. (Good training: egg-shaped objects are not for eating.)
Now...they're overturning the nest box that the majority of them use! What the heck?!?
You took out the other ones they were overturning, so whatever they had been doing on the ones you removed, they started to do it on the one that was still there.
What can I do to stop the destruction?
I would suggest:
--fasten nestboxes to the wall (this by itself might be enough.)
--watch the chickens as they go to roost at night, to see if they have enough space to all fit on the roosts, and if they can get up easily. If you see something that needs fixing or changing, then do fix it or change it.
--consider putting curtains on the nestbox fronts, or use any other method of making them darker, so it is less fun for the chickens to scratch and forage in them.
--consider adding more things for the chickens to do in the daytime, such as a pile of dry leaves to scratch through (chickens seem to really like scratching through leaves, and at this season they are probably easy to get.)
But what the heck are they doing all of the sudden?
Chickens do weird things
I think they are a bit like children or teenagers: someone thinks of a new idea, and they all start doing it. Pullets do pay more attention to nestboxes around the time they start laying eggs, but they also seem more likely to fly out of the coop and try to sleep in trees at that age. It might be part of normal growing up for them, to just try doing things differently and see what happens.