The glass lid to our large stock pot just exploded - literally. Any theories as to how or why?

This is a stainless steel pot with a glass lid that is roughly 20 years old. For a good 15 of those years it went unused because I had move home with my mother post-divorce and we shared most of her kitchen items. However, we’ve been in our home for over three years now, and is is our largest pot that gets frequent use. In the past month alone, it has been used every weekend to make stocks.

My fiancé put the chicken, veggies, herbs, and water in the pot over an hour ago and brought it to a boil the way he always does. He then turned down the heat and let the liquid come down to a simmer before putting the lid on – when he did put on the lid, it was at an angle to allow for ventilation. The metal rim around the glass had some dings in it, so it wouldn’t form a tight seal even it it had been put on flat. After about a half-hour, the glass exploded up and out, all around the pot. Unfortunately, enough glass didn’t clear the pot and fell inside, so we’ve lost everything.

We can’t imagine what might have caused this, so we thought the brilliant physics and engineering and cooking Jellies of Fluther might have some ideas. I’ll post it in Social, so we can still have some fun with it.