Pure chance and there's not really any winning strategy or skill to it other than for the avoid all bananas medal. The only skill-requiring medal.
You're just as well to stay in center as the bananas come down randomly and if you race to one side you may miss bananas coming the other side. Then again since it's random it doesn't make that much difference where you are and if the banana isn't extremely close it's not reachable anyway.
High score medals rely mostly on getting lucky enough a good amount of flashing bananas come (some games only one or two come or maybe none, other games you may get like 5) as well as luck of them being close enough to be catchable.
And that's the game.
You can play flawless and still be nowhere close to the upper end medals only because the game didn't give enough bananas or flashing bananas that game. Relies too much on luck.