What is the most common cause of air bubbles coming out of pool return jets?

It’s almost always a suction side leak. Air gets pulled into the plumbing before the pump, gets chopped up, then comes out as bubbles through the return jets. Common spots are the pump lid O-ring, skimmer weir door, pump basket crack, or loose suction fittings. Check the pump lid first, and use a garden hose test to find the leak.