What do you think would crack an alloy wheel?.........?

My husband has put big 17'' oz wheels on our 1991 fiesta!! the wheels that were on it were 15'' One of the tires was bald so he got a new tire yesterday but every time he went around a corner it made a grinding sound and whenever he went over a speed bump or hole yesterday it made a big bang, Now the tire has gone down and he took it back to the shop that put the tire on and they showed him 2 big cracks in the alloy, My husband thinks the man who put the tire on cracked the alloy with the Machine, And that it has nothing to do with the banging noise grinding or the fact that they are to big, What do you think has actually happened?