Say there is some air in the pipes like in an anti banger device.
Next a poorly operating pump may have variable pressure.
Combined with possibility of some air maybe in hot water heater.
The air allows compression and possible flow.
Variable pressure may have water flowing in and out of water heater
And depending on location of said heater if near beginning of system and connected via a t it may allow bits of hot water yo be inserted into the cold line.
Far fetched but possible...
Tony & Lori
1989 Country Coach Savannah SE