Short answer is water.
Generally wrinkles occur during a heavy rain.
Over filling the pool can also cause this.
Most often in my area of the country it’s from heavy rain, cover pump weight, cover weight and water displacement.
Your cover pump won’t keep up during a heavy rain.
Around the perimeter of your pool under the cover the water will rise.
The orange arrow shows the water at max for cover and pool.
The red arrows show that when that water rises it will go over the side or out a skimmer collar.
Water will also seep behind the liner at the bead then going directly behind the liner.
This can happen to an uncovered swimming pool during heavy rain or over filling can also be a cause.
The ground may be saturated this will also add to the problem.
Un-tamped back-fill around the pool can cause channels for water to stand or flow.
To get a four foot wrinkle would mean you had a water bubble that was that big around.
It pulled the liner towards the center of the bubble.
When the water soaked back into the ground the now stretched and pulled vinyl folded up.
To go back in place it can’t have any water weight on it.
There is not much you can do.
A bigger cover pump would weigh more and could sink the cover additionally.
My customers with safety covers place a cover pump on the first step under the cover to prevent the water rising too high.
Each pool that has this problem chronically should be checked by a pro to see if a solution can be found.
I’ve buried pumps set in barrels, filled with gravel, with holes drilled to pump ground water away from the pool.
These pools were fine all spring and summer every year but every year we had to reset the liner.
After the pump for ground water the liners never floated again.
If you get a wrinkle you need to get it fixed.
Especially if you run an automatic pool cleaner.
The cleaner rolling over those wrinkles will make a hole that is impossible to patch.
If you watched the Beverly Hillbilly’s you knew that cement pond was for millionaires.Mister B.