It may be several reasons
FLASH:
- Unexpected reset during flash erase or flash program operation
- Wear out of particular flash sector = number of program/erase cycles that device’s flash is capable to withstand is exceded (in practice, real endurance is greater than specified minimum).
- Over-programming of particular flash area that is programming without erasing. It is basically software fault.
- Soft error (radiation, electro-magnetic interference, or electrical noise) during reading or executing from flash memory
SRAM:
- Uninitialized or improperly initialized SRAM memory. Note that if SRAM is not initialized fully, even in case there are no data in that area, access to data stored just before the uninitialized area can cause some issues because of prefetching (cache, pre-fetch buffer or instruction buffer)
- Soft error (radiation, electro-magnetic interference, or electrical noise)
Generally for both cases it may be caused by fault due to damage caused by running the device out of spec whatever way.
And finally ECC error may be intentionally generated by ECC error injection.
Here is the link for application note I have posted, I believe it helps you to understand ECC protection. Note that this does not deal with reasons of “real ECC faults”, rather its software detection, correction and injection:
https://community.nxp.com/docs/DOC-103300
