AC leakage from the hot or neutral of the line cord to the box housing or chassis for sure. There are always at least caps from both sides of the line cord to the chassis ground. The chassis of course holds the "F" connectors so it is connected to the shields. I'm not sure if there is also a difference of potential between the shield and center conductor or he was picking up a tingle because he was standing on soil or concrete, but that is the cause.
-Hal
