Why do my toilet valves seem to have started running at the same time?
Run enough water to get the hot water heater to start its cycle, then close everything and measure the pressure until the heater gets to the end of its cycle. If you have a check valve and no expansion tank, this can cause a pressure issue that wouldn't be detected with a quick test. Instead, it will slowly rise as the water heater runs, and then drop back to normal when the first faucet is run.

It's also possible that all the toilets have the same parts and are therefore failing at the same time. But before replacing everything, it's worth it to make sure they are really faulty.