Trying to imagine myself in their shoes, and from what I've learned from talented close friends who delete their work often, if I were to ever do such a thing, I'd do it because:
Catastrophizing.
As self punishment, in some weird, overly-emotional, destructive way.
Wanting to "disappear" in some metaphorical way.
Starting a "new chapter in life".
Some past atrocious experience as a kid/teen, when other things I made got brutally roasted. Been overly self-conscious since.
Maybe they made their works during a year associated with some distressing memories. I've deleted a lot of work that was ok quality, but due to unrelated memories they kept giving me (the people I talked to at the time mainly), I hated them.
A friend of mine makes some amazing work, but is a perfectionist (due to trauma), he won't stop comparing his works to other people's. But he has a mental illness he's struggling with daily that heavily impacts his views in this case.