1. Exploits. Like you mentioned above.
2. A fix that was scheduled for a later patch, but was already coded up, so it got thrown in a patch early. When this happened, the start of testing in QA would have been delayed for some reason (maybe the release candidate build was broken for several days, or the release candidate was missing stuff that should have been delivered a week ago). A dev lead or the producer might look at a list of bugs that were already marked “RESOLVED FIXED” for the next patch and say, “When you get a sec, check these half-dozen changes into the release candidate tree before QA starts testing,” because everyone likes it when more bugs than expected get fixed ahead of schedule. The community people who are responsible for posting a weekly list of upcoming changes might not be told, or might only find out when the official patch notes are generated the day before the patch, so the players don’t find out until the last minute. Cue player uproar and conspiracy theories about a “stealth nerf”.

The second option leans heavily on “lack of internal communication” like you mentioned, but is minus the internal warfare in your hypothetical scenario.