When I flush my toilet air backs up through the tub drain

Hi Mark,
You have a partial blockage that's blowing back through your system.
You flush and the discharge hits the blockage and bounces back sending a bubble of air ahead of it. This is what comes back out of the tub trap.

The solution would be to snake the lavatory roof vent. Put out enough snake to reach the base and 20 feet more. Good luck, Tom