Indeed, most likely cracked then torched through by the exhaust gases.

As to the cause, rather tricky to say, I'd rule out it solely being a metallurgical issue, due to the fact the rest of the plate is not compromised. I'd go for resonance generated metal fatigue, aluminium castings can be quite susceptible to that kind of thing, & it would appear that part of the casting does over hang from the studded/bolted area which would allow it to behave in a tuning fork type manner.

Although fordsportjay's theory is quite credible too.

Bad show all the same, one would've expected the supplier to at least offer you a discount on a replacement.