Lots of great theories here! Could a piece of solids (pea stone?) have been pumped into the system and, not being able to escape through your slits, got lodged at that one spot and eventually built enough pressure to rupture?
What's feeding your bog? Where is the water being pulled from? Do you have pea stone in your pond?
Just another theory Honestly, seems unlikely given that sch 40 pvc is rated for, I think, 450 psi and most pond systems operate at a psi less than normal house pressure, which is usually under 80. But I wanted to contribute something!
You don't happen to pump 150ºF water through your system do you? Haha.