When a user's account is deleted (for any reason), all their negatively-scored posts are automatically deleted by the Community bot. From this post:
The system will automatically delete any question (and its answers) or answer with a negative score when its owner’s account is deleted.
On the main site, this is all well and good: if a user had some crappy posts, no point keeping them around when the user themselves isn't around any more.
But on the meta site, it can cause problems. There, "negatively scored" isn't the same as "crappy"; and users such as spammers who get quickly deleted don't often acquire enough rep to post to meta. Someone might have posted an unpopular suggestion or feature request which, although declined, is still useful to link back to; this should still be available to the community if the user quits.
(The example that motivated this was a user on Science Fiction & Fantasy, who was once a very active and valuable meta contributor but recently deleted his account over accusations of mod abuse. He'd posted several answers to an important meta discussion on the site's scope, and three of these were downvoted, reflecting the fact that those topics were decided to be off-topic. I only recently discovered that these answers had been auto-deleted, and had to undelete all three of them. There could be many other useful negatively-scored posts by this user which have been deleted and which I haven't yet found to undelete. This is not a good state of affairs.)