All foods are slightly radioactive.

Bananas are especially radioactive due to the potassium like geiger counter register radioactive.

Also people like to ignore that the body will keep the amount of potassium in it constant. So you have always the same amount of potassium radionuclides in you.

This leads to some people underestimating the effects of radionuclide intake.

A body with properly functioning kidneys will keep the amount of potassium in it constant

The "some reason" is that it helps to put into perspective some radioactivity fears.

"Working for a week at the Fukushima plant gives you the banana equivalent dose of one hundred bananas (100 BED)", or whatever.