I have the same thing going on. From my research it could be 1 of 2 things.
1) The consistency of the banana smoothie is too thick and your body thinks it to be mucus and intern begins to flush all the mucus out of your system (through sneezing)
2) A few doctors have told me if you start sneezing you may have a very slight intolerance for bananas. When I first heard this I was sort of angry because I'd hate that I couldn't do this sort of diet without bananas. As time went on I realized that its far more likely that it is #1 because I have sneezed less when I add more water to the smoothie and also when I eat bananas outside of a smoothie I do just fine no sneezing at all! Hope this helped!