I am in the process of changing my auto insurance and I got insurance with a new company and called to cancel the insurance with my old insurance company. They told me I have to fax them the proof of new insurance before they can cancel? Is that true? What’s up with that?
On my policy I have homeowners insurance and two cars, one is paid off and another one is still not paid off completely, if that makes a difference.
I feel that something fishy is going on

Most likely if you fax them proof of your new insurance then they won’t have to report you to the state for canceling your auto insurance with them.
Having auto insurance is to protect the rest of the world from an accident that you might create so it is required by (most?) state laws.
However, to be sure, it is simple to ask them why and and get their answer.