I'd wonder if the air vent line(s) for your plumbing is what's clogged. The water can't flow down unless the air can escape.... and if there isn't good vent, it'll glug glug down, like a bottle turned upside down.
That's what Steve said. I asked him "Where are the air vents?" He said "I have no idea."
But that is just what it seems like...because when the tub releases, it's like it pulls a suction on it...it just sucks down quick!