Have any of you ever had this happen? or heard of it?

I take great care with my stones, and am baffled by this.

The glass backing plate is intact and undamaged. I never soak the stone. when I flatten, I rub the stone on top of the DMT, using only finger pressure to swirl it around. Never DMT on top of stone.

I honed a blade, all was well, 30 minutes later, I'm honing a second blade, and feeling an edge.
Crack simply seemed to appear, not there one moment, there the next.

Thoughts?