- Why was it locked? I think putting it on hold is plenty.
- How did the Community user lock it? I thought only human mods can lock posts.
When a post is migrated to another site, the Community user locks the post to prevent attempts at reopening, votes, and comments (and really, the only way to get to the post is to ?noredirect=1 in the url).
Eventually (30 days), the post will be deleted if the migration isn't rejected on the other site.
The goal of the combination of the lock and the migration is to direct all future activity on the post on to the other site rather than having it fracture between two different sites.