The question was;
" Hi Guys,
Does anyone have any idea what the possible cause of this is? "
Detonation may not have actually done all that damage, but I guarantee you it was the cause of the damage. Below is the head I pulled off the first rb26 I bought, and it was trashed just like OP's. When I opened it up, most of the chambers looked like this and there was little BB looking bits of melted alum in the cyl bouncing around.
The cause was severe detonation that melted the piston crown / ringlands, which caused pieces of broken rings/ ringlands/ moultin alum to help do the damage.