When the pipe is full the air can just push the entire mass of the water, but at some point some of the water shears off from the rest of the water mass because of pipe friction and additional friction/head lose at any connection or fitting.  This water seeps down to the bottom of each loop but doesn’t fully or probably even block 1/4 of the pipe surface area, the air just passes over the top of this water.

Once you lose the ‘seal’ of having the pipe completely full of water you have to generate enough friction to lift the water up past the same sort of water trap you put at the outlet of your air tank to keep your shop air lines dry.  Which you can’t do at 150 psi.

So now there is a bit of water at the bottom of every roll of pipe, and when You pry off the cam lock, apparently with 150 psi on it which seems, ummmm, fun I guess and explains why the cams were tight, and the air starts to move out the open end.  The velocity of the air creates drag along the water’s surface against the air, which is enough of a friction force to drag the bit of pooled water along in the first or second loop of hose on the reel even if it can’t push it out through the other dozens of loops the other way.