So, maybe (again, me being a layman) what happens is that usually gamma rays hit a DRAM cell, but haven't imparted enough energy to cause a flip. A millisecond later the cell gets refreshed erasing what little influence the gamma ray had. No harm done. A flip would only occur if enough particles hit the cell within the refresh time frame. That's of course possible, but more rare.
Contrast this with processor cache. On-die cache is most likely SRAM, Static RAM. It doesn't get refreshed. So the slight voltage errors caused by gamma rays can slowly build up over time.
Perhaps this normally isn't an issue, because even though the cache is SRAM and doesn't get refreshed automatically, it'll get "refreshed" by virtue of being cache. i.e. as long as the processor is busy the cache is constantly getting re-written with new cache lines.
But that won't hold true when the processor is asleep. The cache will be sitting idle, making itself susceptible to accumulated charges. Thus the likelihood of a gamma flip is greatly increased.
All of that crude logic aside there's one caveat:
> he workaround was removed once the problem was fixed in microcode or in a later processor stepping.
So ... either everything I said is a load of bollocks and actually this was a processor bug that some CPU engineer mistook as gamma flips, or maybe my theory is correct and they changed the CPU to occasionally wake up and "refresh" its cache automatically.
The mystery remains...