I always thought it was related to the fact that the phone is pumping out alot of RF energy.
I remember being shown how a light bulb, with two wires, one pointing up, and the other pointing down (i.e one wire connected to the + and - on the bulb) then held near a transmitting CB aerial. Quiet amazing how the bright the bulb got.
A bit more tuning of the wires soon showed how much energy was being pumped out of the CB. Put it this way, wouldn't like that much potential between something that is grounded and something that isn't, and just enough space and vapour.
It is quiet and Interesting experiment.