The last thought I would have is creep - ever-so-tiny flexing that is causing the glass to fissure.

If you wet out glass on a very thin piece of plywood and after it hardens up, bend it a bit the glass can start to show. It would only happen on epoxy that has totally hardened for many, many months, maybe years. That's why it may happen after years of use. The epoxy starts to get brittle hard and looses a lot of its flex.

Post a picture if you can. maybe others might have other ideas.