If it's only 2 fish out of an entire group, it's probably nothing directly related to the water change.
Since you mentioned you have city water, I noticed a couple times when on it the water department would accidentally have spikes on occasion in the nitrite level, amount of chlorine, etc. IIRC, when I was in Ann Arbor I was able to check their website to get statistic after the fact ... never knew before it'd already happened though obviously. When I'd do water changes I'd always use more water conditioner than necessary as a precaution.
I suspect it's aggression related. If you watch the tank long enough, you might be able to confirm it. I also have always noticed that my cichlids get a little more rowdy after water changes, so perhaps that's why it correlates to a water change in your case as well.