Why would water pressure at my faucets change after turning off the water main for some time?
Debris. Mineral buildup, sand, etc.... You loosened it up, when the water came back on it pushed it into your valves and aerators. Pull the aerators off and clean them out. Of it improves, no need to pull the valves out. Of better, but not the way it was before, turn water off statue faucet and pull the valves out and clean them.   I just did the same thing 2 days ago and have 1 more faucet to clean the valves in.