Two thoughts in case unexplicable stuff happens again:

• SMART doesn’t detect all possible errors, especially not on SSDs. (It was originally designed as an early-warning system for mechanical failures on spinning rust drives, and later somewhat expanded.)
• A faulty RAM chip can cause all sorts of subtle (and not-so-subtle) mayhem but is often overlooked as a possible cause for disk troubles. If in doubt, MemTest86 is your friend!