Posted By:
Anthony N.The highest grade PSA will give a cracked card is a 3, assuming they see the crack.
It's not unheard of for these to crack after grading, sometimes years after. Cold weather speeds up this process.
Even more painful than Kelloggs cards cracking is '68 Topps Test 3D's doing it. I've seem some very, very expensive examples do this, and it's brutal.
I strongly suspect your 8's were cracked after they were graded, and will only continue to do so.
If you search the CU set registry board that are tons of threads about this- at one point the '70 set was hugely popular.