Could be any number of things :
Undervoltage, chip instability period (over the maximum output of chip), ram timing problem, ram voltage, ram frequencies....
Thats why it's best, when oc'ing, to find each components level of stability, so that when it does crash during the oc'ing of your system, you know exactly why.
For example.. We isolate an OC problem and it happens to be voltage. As long as it's "stable" it doesn't matter what voltage it's at? Assuming temps are within range.