The only rumors I've seen are generic guesses: something AMD wasn't screening for, maybe coincidence, or maybe detected after the Intel mess sent AMD's QA teams scurrying to make sure they don't have any similar issues.
It could even be a combined issue with 3rd-party motherboards with AMD's new chipsets, the combination of which they wouldn't have been able to thoroughly test much earlier.
Isn't RUMINT on the Intel problem that, even though it's nominally a chip problem, it may occur primarily due to motherboards not following guidance? For instance, if a spec says the chip and mobo should lower max voltage by 100mV under certain conditions, but the chip still sometimes requests the full original voltage under those conditions, and the mobo provides it, whose fault is that? Maybe not exclusively Intel's, depending on how the specs are documented (a classic should vs must issue).
It seems likely that small process sizes and pushing the limits of performance are going to cause more problems like this Intel one. Notice Intel didn't have this problem before they had to push their chips to compete against the high-end Ryzens.