Yes. It's the potassium.
"Bananas happen to contain a relatively high amount of potassium, but only a small fraction of that potassium is radioactive. ... a banana contains, on average, 451 milligrams of potassium. Fortunately for us banana-enjoyers, only 0.012% of naturally occurring potassium is radioactive."
That's pretty interesting. I had a friend that had aids and he would have to get potassium shots sometimes. I wonder if the radiation was some kind of treatment for the disease. Kind of like how they use radiation to kill cancer