Liner wrinkles

I have an above ground pool, 18x33, and started to notice my liner at the bottom of my pool is starting to "wrinkle" in places.
What could be causing that??
Tunneling ants???

More likely shifting ground. Much ground water?

Wrinkling of vinyl can also occur from low pH conditions such as using granular Trichlor that sits at the bottom, but I presume you use chlorinating liquid or bleach as your primary source of chlorine and haven't added any such powdered chemicals to your pool, correct?

Pouring Muriatic Acid directly into the shallow end of your pool can also cause wrinkling as the acid attacks the liner. Always dilute it in a bucket first before adding it to your pool.