If tightening the QR did actually fix it, it may be a broken connection in the hub, the Shimano hubs have the "earth" connected to the bike chassis. So there may actually be a connection being made because a QR can measurably compress an axle (which is why you set wheel bearings with the QR done up). Alternately , it could be the "positive" (hah AC, but you know what I mean) contacting the chassis, and the QR pressure uncontacts it. Best way to test would be with a multi meter, during a fault time. The earth terminal should be connected to the axle with zero resistance, the other terminal should have some resistance between the other terminal and the axle.