5th upstream channel?

But this is out of nowhere. I've always been able to see 4 upstream channels. Now I see 5

Adding upstream channels happens out of nowhere. It's a good thing.

automatically by my modem itself?.

What causes it?

The more bonded channels you have the better. Nothing to worry about. At the same time consumers are not in control of how many bonded channels we get. Its a combination of your modem's capabilities and your service area's capabilities and Cox' networking decisions. We are just along for the ride on this one.