I'm 90% sure that the original meaning of the word "hook up" involved an actual, literal hook. It was part of the mechanical system for bringing electric power into the house from a utility pole. A supporting cable, which didn't carry electricity, ran from the utility pole and was attached to the hook. The hook took the strain of the cable. To bring electricity to a house, it needed to be "hooked up." So "hook up" came to mean electrical connection, then any kind of connection.
I say "90% sure" because I haven't been able to Google a confirmation of this, and unfortunately I can't find a picture of an old house with something that looks like a visible hook.