Reading more on this, it turns out Skynet 1-A's payload comms amplifiers died 18 months into the mission, but not the satellite bus itself or its built-in communications. So it was a mobile, controllable satellite that could be moved around at will for any number of reasons (practice? experiments? simple orbital science data-gathering?) without worry about compromising its mission, since that had died with the main amplifiers. I'm guessing no one remembers why it was moved because no one simply cared that much anymore.
Well, from the above plot, we know it had been moved by about 1978/1979 at the latest.
As for the "Why?" of it, yeah, probably lost to time but overwhelmingly likely it was for fairly mundane reasons, given the thing couldn't carry out its intended operations. It was the satellite equivalent of a busted cell phone without service that won't charge anymore that someone gave to a kid to screw around with until the battery finishes dying.
Some solid reading here: The Curious Case of Skynet 1A