Unbundling Ground Wire make unconnected wires show hot
Edit: OK so it's not the light.  But it's a ground fault somewhere. One presumes it is in the circuit, but grounds can be crossed over amongst circuits in the same panel, especially in a ground retrofit situation.  So it could be anywhere wired which feeds the hot ground wire.  

There should be no intentional current whatsoever on a safety ground.  (though there can be micro-currents due to coupling or the tiny, tiny amount of leakage which is allowed.)

Unexpected currents like this are why grounds must be pigtailed, so you don't have to sever a ground to replace a device.   

Obviously, bootlegging neutral from ground is the picture postcard definition of a ground fault.  

If your previous homeowner did it one place, good chance he did it lots of places.  

It's hot because when you interrupt/sever a current return, current is no longer able to return.  Normally it flows freely  back to the panel, and that pulls it down to near earth voltage.  But current can't go any further so it's "piling up" right there.