Why is my hot water circulator causing there to be hot water in hot and cold pipes?
Somewhere at the end of the line there should be a valve that bridges the hot and cold water lines, which is usually under a sink. If that valve fails or is installed backwards then you will get excessive hot water in the cold line.

To test the valve:


Find the sink with the valve under it.
Close the cold water valve under the sink.
Open the cold water faucet.
Water should SLOWLY come out of the faucet. (If it comes out quickly, you have a bad valve.)
When the water warms up, the flow should slow down and eventually stop completely. (If it does not stop then you have a bad valve.)