Sounds like Brown Rot - usual cause is damage to the skin of the apple. Possible damage causes are bird peck, codling moth, apple sawfly, irregular water supply causing fruit split. You need to establish what the original problem was - you can check for moth infestation by cutting an apple in half to see if there's a tunnel inside it.
Brown rot does sometimes occur spontaneously, usually because there are too many fruits and they're touching one another, and the weather is very damp, but its usually a secondary problem.

Thank you