Why do strawberries do this when they spoil?

It's not the strawberries doing that it's the fungus growing on them.

If you find white fluffy stuff on your berries that looks a bit like cotton candy, that is mold. Mold is a fungus with spores that feed on the berries and grow thin threads that can look like fluff or cotton. This particular type of mold is common among fruits and is known as Botrytis fruit rot or gray mold

Their moisture content.