Unless you are using an indoor antenna, somewhere the cable from
wherever is grounded. It is obviously not grounded at the same place
as the electrical ground.

In cases like this, chances are that the cable's nearest ground is
either out at a street-side service pole, or a curb-side vault, or a
neighbor's house. There's all kinds of opportunity for current loops
between the cable ground, house ground, and neutral.

You can actually build one, in case you would rather have that isolation transformer for other uses. If so, I'll look into the best/easiest way to do
it, but if you are happy now and this is all academic you can just leave it as it is. However, your equipment is no longer grounded, which should not be
a problem.

http://flynwill.rosshay.com/Electronics/antIso/ shows a nice way to
rebuild a standard cheap 75-to-300-ohm balun, to turn it into an
isolated 75-to-75-ohm unun. The result is probably rather similar to
what you'd find in a commercial 75-ohm coaxial groundbreaker.

(This very problem was the source of a discussion on the FMTuners
mailing list last week, and that's where I cribbed the reference to
this particular DIY project.)