how does bronchitus start off as a viral infection then progress to a bacterial?

It's called superinfection, when you get one infection on top of another infection. Bronchitis usually happens after a bad cold, in which the influenza virus destroys the hair cells in your respiratory tract. Without these hair cells (cilia), you can't swept up stuff that enter your respiratory tract, such as bacteria, so they come in and infect you. Having a cold also makes you weaker and more susceptible to other infections. So it doesn't so much progress to another infection as it led to a second, separate infection.

Apparently this is really common. I have had the same situation happen to me. I started off with a regular head cold. But because this caused my sinus cavities to become inflamed it turned into a sinus infection which then progressed into bronchitis