very very odd :o, my metallurgist at work would love a challenge like this, but would probably teel you that this is down to a previous defect ( as mentioned before this looks like either the centre has been heated / had its material properties changed ), or that a stress fracture has been created a while back, which has just grown into a full crack.
Either way, I'm glad to see you walked away, this could have been a fatal incident.
You REALLY need to find out who did this & put an end, as if someone thinks they can repair wheels ( if indeed it was a repair, which I reckon it is ), they need to be stopped, as this WILL kill someone else

Agreed with hard alloys, they don't like being heated in a zone, the only way is to stress releive the whole lot, costs a lot, as you need to know the material, times etc to heat, then controlled cooling etc...
I would strongly advise you to check all the other wheels ( especially the rear faces, as these will show up cracks, as they are rougher, which is where a crack may initiate ( rough surfaces )