Possible causes:
1. Backing plate is too big....as David Fermani mentioned.
The edges of the plate is biting into the foam, leaving a gouge that eventually worsens.
2. Too high speed.
3. Too much pressure.
4. Stopping the machine from high speeds, with the pad free-spinning. Pad shd be in contact with paint surface, until the pad stops completely.
5. Yanking-off method. Since your backing plate is big and its edge rest on the foam (and not the velcro), be careful when you pull off the pad from the backing plate. Don't yank the pad off. Using your fingertips, slowly "peel off" the edge of the pad from backing plate's velcro, and pull slowly to get the pad off completely.
No5 was the main reason my pads had the problem as yours years ago.
I've lectured my boys on this, and to this day, my pads are fine.
During work, if I need to clean the pad, I remove it, and put it on a rotary, and clean it with my Lake Country Pad Washer (that bucket thingy). Then, rotary spin it dry and air dry, while I use another pad on the DA.