Oi Rob.........
Is that a cutting disk? and you where grinding? Thats why it exploded!
Cutting disks cut via the edge, grinding disks grinds on the flat and the edge you may have caught the flat side of the disk but not known and then it ground itself down!
I did this the other way round many years ago, tried cutting a S2 chassis up using a 9in grinder with a grinding disk.......it took so long........I only reaised it when I changed it!!!