Hmmm. You would think that for water to get in, air would have to escape...unless...the water is going in under pressure - ie when you are going fast or if the hole is at the bottom of the keel (where there is a bit more water pressure - albeit not much more).
Can't you tell I used to be an engineer? I might have to pull out a slide rule on this.
So, continuing with this hypothesis, the pressure forces the water into the hull through some opening, but then when the pressure is gone, the raised pressure inside the hull (created by adding water but not removing air) is still not strong enough to let air escape (don't ask me why...this is just a hypothesis). Soooo...when you open the plugs, the boat puffs out at you.
You say it happens when you are flying a hull, do you get water in when you don't fly a hull (ok, we know that could ONLY be on light air days
Seriously, though...how much water do you get out of each hull? How long would you be out on the water for this to happen?