Question

what does it mean by to go bananas

Answer 1

To "go bananas" is an idiom. It is usually used when someone does something out of the ordinary. 
Hope this helped you.

Answer 2

To go "bananas" it means to go wild. Or crazy.

If you've seen monkeys in cartoons, when they want banana's they go wild and some refer to it as "go bananas" !!