If people are using single handle faucets and they open the faucet and try to get warm water, the cold water will back flow.
Now if the issue is where no one is running water and you are getting backflow, then the trouble is with a failing single handle cartridge. Very common in Moen. I have been on service calls where people complained that when they turn on just the cold water, hot water comes out first for a few minutes. I replaced all their Moen cartridges and problem solved.