That's because of the difference in the metals. The electrical charge is from the difference in the galvanic series of the metals involved, they form natural batteries in the solution, which is why the immersion deposits are thin, they're not auto-catalytic they require the exposed substrate to deposit, once the copper coats the surface of the steel the deposit process stops. If you apply a DC voltage though the process will continue, until most of the metal is deposited onto the substrate. But not evenly.
I worked in the metal finishing industry for 10 years, this is a cause of failure in our Zinc plating occiasonly when we overload our muriatic acid cleaning tanks with brass parts. The line had a fix cleaning cycle which couldn't be changed, ordinarly brass isn't acid dipped for this very reason. After the acid becomes saturated with copper it will immersion deposit on steel subrates, but it also brings a bit of a smut with it which really messes with the electrolytic zinc plating process causing all sort of problems because the current density changes from teh copper substrate relativer to the normal steel one. The most common outcome was a black appearance to the zinc at the high current density areas (edges) of parts.