Why do some high karma Reddit accounts suddenly disappear without explanation?

Occasionally established accounts vanish completely after years of activity. Do people eventually get tired of maintaining online identities, or are privacy concerns increasing?

Privacy concerns about what? Sometimes, they get sold so they get flagged. A lot of farms on Reddit.

That's quite normally

To prevent scam Reddit start rolling out small updates every month during which Reddit filters are highly activate and scan the Reddit database to find the unmoderated subreddit and accounts that are doing any kind of spamming and too much promotional activity, if any account got caught up by the Reddit filters during that time then Reddit ban that account or subreddit without any warning.

I think it's usually a mix of both. Some people simply lose interest and move on, while others become more concerned about privacy and decide to delete years of posts and comments. I've seen plenty of high-karma accounts disappear even though they weren't banned or inactive.