In my case it was the ladder. The ladder was rubbing up and down on the side until it broke open a hole which then just blew out with the force of all the water.

I just purchased a new liner ($750) and it will cost me another $400 to install.

I will stay away from drop in ladders and will now try drop in steps and hope for less abrasion of the walls.

All this for a 2-yr old pool.