what causes an apple to get brown?

As time progresses, apples rot and go bad. Bacteria and chemicals cause them to get mushy and gross. Try eating apples earlier, in the morning perhaps.

Bruising it or hitting it too hard.

Its wild yeasts in the air they are attracted to the ripe fruit and the sugars it's how alcohol was discovered enzymes whatever a bad apple can spoil a barrel full you know that,  elephants get drunk eating fallen apples in Africa 

Oxygen air makes the food go off colour

There's either a bruise under the skin or if the brown parts are scattered like little dots the apple tree was suffering from some mineral deficiency while the apples were forming.  

For the people not reading the question properly the apple was already brown under the skin before the asker bit into it, so the answer isn't oxidation.

oxygen reacts with the enzymes in the flesh when the skin is broken.