Hello,
There are many reasons.

First that comes to mind is disease (mainly fusarium). This can cause a melon to "mature" faster than normal. Also too much water, genetics, environment can all play a role. There are many varieties that are fully mature and will slip but will not show any color change. Also a high heat spell can "mature" a melon quickly.
I have one question for you. Is the vine really strong?

Hope that helps,
Jonny

By strong, if you mean healthy, the answer is no. I have zucchini, butternut, cucumber and cantaloupe not too distant from each other. The zucchini is the only one still vigorously growing and producing. The cucumbers as the butternut produced extremely well but also look like they are at the end of the season. We had an extended dry period and I do not think I over watered them.

John

Here is a link that might be useful: Johns Journal

I would ask what variety you are growing. Superstar can soften quickly.