Bananas are recommended to help relieve gastric pain and strengthen the digestive system -- in humans. It also can help relieve constipation.
I have just discovered, while it's less common than lactose intolerance, some people experience gas and bloating from fruit because their GI system doesn't break down all the sugars in fruit (fructose and sorbitol) properly. This is new news to me, though it makes sense then that perhaps some rabbits may have the same problems?
In rabbits, sugar overload can cause poos to get soft. And any individual rabbit may not tolerate a particular green or fruit that most other rabbits have no problem with. It could be that your rabbit that gets soft stool from banana, just happens to not tolerate banana well. (It's either that or the sugar overload?)
Regardless, I consider fruits (any fruits) to be treats only and limit them anyway.