People are going back to revisit things they think they deleted a long time ago to find it not deleted.
It's possible that those items were deleted, and then suddenly undeleted over the past few days as Reddit plies its nefarious plans. Far more likely, they were never deleted in the first place, or had the same temporal issue and reappeared a short while after deletion, but long long ago. The person thought it was deleted, but it wasn't for the same reason people are finding deletes not to stick now in some cases.
The obvious benefit would be to artificially keep interaction rates high by "grave robbing" old comments and accounts.
"Nobody is ever going to notice, those accounts are long dead!"
The metrics you use to attract investment are things like monthly active users, and whether a comment was deleted after it was made doesn't affect that at all.
No database shard would ever have been left that far out of date due to high load.
The only Hanlon's razor I can imagine here is somebody wrote a query to try to undelete something small (intending a single user or something) and forgot the WHERE clause and started to undelete everything. And didn't realize until the query had been executing for several minutes.
Yikes.
(I do actually think this is the most likely, not because of spez's motivations, but because there's zero benefit for Reddit here. It might make sense for them to force subreddits back to public, but undeleting old comments has no strategic value right now.)