Dual hose portable AC unit creating negative air pressure
I've heard that a lot of portable ACs are cheaply constructed and may have significant gaps in the construction of the — not sure what to call it, the condenser compartment, the hot side. Such gaps, if larger on the intake side of the blower than the outlet, will pull in more air than they leak out and produce a net negative pressure, like a simgle-hose unit but less so.

If you can find such gaps, sealing them up should lessen the negative pressure effect.

However, also remember that a door is a fairly large surface, and so a small pressure difference creates a lot of force on it. The amount of air moving may not be all that much; you've got a sensitive detector.