hydrostatic pressure

[JohnT]

Was your pool empty at the time?

[CarlD]

Did it blow out or blow in? Was there an increase outside pressure or a drop in outside pressure?

[mustBnuts]

If it was full at the time, I'd say you had some erosion from underneath the pool. If you pumped ground water from the sump pit located under the pool, some sand may have been washed out with it, leaving an area with no support for a pool full of water.
If it was empty at the time, I'd say external pressure from ground water was to blame.