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

[golf nut]

Silly question I know, but if any of the new plumbing is going up and then down you will certainly trap air in there and it will be forever before it is evacuated.

[Pescador]

No they are pretty straight runs, everything would working fine but if I opened the valve to the skimmer past a certain point it started blowing bubbles to the main tank. From a hose barb on the teed line to the skimmer is about 24" of clear hose so no trapped air there.

[golf nut]

Pics would be helpful.