Depends on D19503. Ref T13151. See PHI719. If you have something like a script which updates an object in a loop, we can end up queueing many search reindex tasks.
These tasks may reasonably contend for the lock, especially if the object is larger (lots of text and/or lots of comments) and indexing takes a few seconds.
This isn't concerning, and the indexers should converge to good behavior quickly once the updates stop.
Today, they'll spew a bunch of serious-looking lock exceptions into the log. Instead, just yield so it's more clear that there's (normally) no cause for concern here.