The heated water more easily leaches the copper from the pipes.
Quoting from the web....
How can I reduce my exposure to copper?
Copper works its way into the water by dissolving from copper pipes in the household plumbing. The longer the water has stood idle in the pipes, the more copper it is likely to have absorbed. (Newer homes with copper pipes may be more likely to have a problem. Over time, a coating forms on the inside of the pipes and can insulate the water from the copper in the pipes. In newer homes, this coating has not yet had a chance to develop.) Thus, anytime the water has not been used for more than six hours-overnight, for example, or during the day when people have been gone to work or school-it should be cleared from the pipes before being used for drinking or cooking.
This can be achieved by letting the cold water faucet run until you can feel the water getting colder-usually 30 to 60 seconds. This must be done before taking drinking water from any faucet in the house.
In addition, hot water dissolves copper more quickly than cold water; as a result, water to be used for drinking or cooking should not be drawn from the hot water tap. If you need hot water for cooking or drinking, take water from the cold tap and heat it. It is especially important not to use the hot water for making baby formula.
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