What's actually happening to an apple when it "bruises"?

Answer 1

When an apple bruises, cells split open and chemicals called phenols, within the cell oxidize and turn brown. Phenols oxidize even faster in the presence of an organic chemical called ethylene, a chemical produced by bananas during ripening.

So if you were to place a banana next to an apple, once you have initiated the bruising process, the ethylene gas being produced by the banana will speed up the chemical reaction producing a more pronounced bruise. You may find large "bruise" marks within days.

Answer 2

Where it bruises it starts to die and oxidation.