Basically I have way too much mana and I'm having far too much fun casting Earthquake every turn. Unfortunately my gleeful destruction comes to a sudden and abrupt end because every now and then it will cause a hang.
I had a look in the debug.err file to see if there were any messages (like, Debug: planetary destruction commencing) but at the end it just says:
Debug Message: Spell cast: Earthquake at 49, 54 by Cuessa (unitID 257).
It's virtually the same message as on the previous two turns.