What is causing air in the hot water line?

Normally dissolved oxygen

Hot water causes thermal expansion which leads to this issue and can easily be fixed by installing an expansion tank. (which is code in many areas) Also if your hot water heater is "turned" up to high or malfunctioning the water can boil and cause condensation within the system thus leading to air in the line.

All fresh water contains oxygen as water is heated it gives off bubbles like pot of boiling water