Pump puzzle

[Pescador]

I'm running a Blueline 100hd return pump (1900gph) and had a tee to a manifold to feed my various reactors.
After doing a recirc mod to a CS8-3 I supplied it from the tee as well.
When I diverted too much water from return, it started blowing air bubbles into the tank.
I just can't figure out where the air was coming from. I'm thinking it's some part of fluid mechanics and I must have skipped out that day.
I've since added a separate Turbo Sea pump to run all the aux equipment and all is good (except the power bill) but I'm still puzzled where the air came from.

Thanks