So I was running this 400w probe start MH/MV wall pack with a GE mercury lamp as normal, and probably after about 15 minutes, I heard a loud pop/thump and discovered this. Anyone know what happened? Nothing touched, dripped on (which might cause thermal shock) or moved it in any way.

Fracture from internal stress in the glass. Sadly happens from time to time.

When Glas cools during manufacturing sometimes when done to fast or small internal stress builds in the glass. At some point it can happen that the glass the fractures.
Sometimes the stress sources, for example with thorn fluorescent lamps, were the end tends to fall off.

There is sadly nothing we can really do about this.

 If it was running with the glass up , a lot of heat is ending up where it wouldn't normally be which could've caused a hot spot which created enough stress to crack the glass . One tiny manufacturing flaw combined with uneven heating of the glass and BANG , you have a crack .

A similar example of damage to lights when run in a different orientation is when the Sylvania Urban 125w MV is run bowl up, it'll burn right through in an instant!