Banana peel bruises after a hit
Fruit is discolored in the first place because, when certain compounds are altered (oxidized), they turn brown. 

In a banana (and most fruit) those compounds are phenolic compounds, and polyphenoloxidase is the thing that oxidizes them.

In an undamaged banana, the phenolic compounds and the polyphenoloxidase (that can turn them brown) are physically separated.

When the banana is crushed/dropped/etc, the structure of the banana is damaged so that the phenolic compounds and the polyphenoloxidase come into contact.  The polyphenoloxidase oxidizes the phenolic compounds, and you get that brownish color.

A nice summary (which links to a more detailed reference) can be found here: https://www.scientificamerican.com/article/why-does-bruised-fruit-tu/