Don't know, maybe i'm wrong anyway, not an electrician.
Maybe it's just static electricity which could be normal, i really couldn't tell.
On the other hand ; i should feel some sparks now and then if my case had that problem to.
I once bought a second hand pc and the gpu wouldn't work, tried everything but no luck, brought it to the store where they found out that at the back of the pc with all the connectors for internet, usb etc there were some parts of the metal ( clips or something, don't know exactly ) bend inwards and made some kind of short circuit, not so bad to shut the system down but bad enough for the gpu not to work, after fixing that that it worked perfect.
I don't say thats your problem to, but just that it can have lots of reasons and impossible to tell without physically seeing the pc.