How is NAND flash going to die when overheating
A few things I can think of.


Chance for memory to be erased (poor data retention).  In flash memory, charge stored in the insulating gate of a MOSFET acts as the memory element.  Under high temperature this charge leaks more quickly.  From ti paper: "with sufficient thermal activation, all bits could lose their charge".  Also from the same paper, gate oxide damage.
Trapped charges injected into the oxide due to the high temperature will build up.  Over time this will add up create a voltage offset on the gate (making FET more leaky).  This could create a situation where the FET is permanently on (unless the gate is driven sufficiently low) if enough charge builds up.
Shorter lifespan in general.  Heat causes a lot of processes to speed up exponentially: (Arrhenius law).  Thermal expansion can cause peeling of metal vias from their respective layers or a host of other problems.
Possible complete failure due to thermal runaway (unrecoverable). 
Higher leakage overall.  More power burned.  From pdf on leakage, in 65nm process: leakage at 0C is ~10x less than 40C, which is ~10x less than 100C.  
The circuit will run slower than expected, due to more collisions of free carriers.