Can anyone explain this?

I’ve had a Rega turntable and Grado cartridge combination for many years. Both are known to cause hum.  I’ve lived with this hum, thinking there was no way to alleviate it.
  A friend suggested I get a transformer that would eliminate it.
So, I ordered a Jensen ISO-Max transformer which arrived today.
Hooked it up.  No better.  The hum remained.
I removed it and plugged the cables into the preamp as usual.
THE HUM DISSAPEARED! at normal listening levels.
Now, through the years I’ve hooked up and unhooked the turntable several times in different positions.  Always the same hum.
Can anyone explain this phenomenon?