Why does hot water now come out of the faucet when I turn on the cold water only?

The faucet is a bathroom faucet (one spout, separate knobs for hot and cold water). The electric water heat (2 1/2 gallons) was changed yesterday by plumbers. I almost never use hot water. Every time I turn on the cold water, hot water comes out for 10-20 seconds. What happened? How can this be remedied? Thanks.