Hmmm, I have never experienced this myself, on any wing.
But, if choppy waves, I have, the broken small wave tops sucks air under the water, which causes your wing to ventilate, even if quite deep down.
And on my shortest mast only 83 cm, it happens on occasion, simply because it is so short that it WILL suck air down sooner or later.
These are my best guesses
Some say they have experienced air gets sucked down along the mast, and causes the wing to ventilate.
Not sure I believe this is true though, it might, but I am sceptical, at least when talking about freeride speeds. Maybe at racing speeds, this can happen.
You ride a very short mast - it might also be the reason as said, so you still get some air from close to the surface sucked down on your wing, without you knowing
Does it happen in low winds almost flat water surface too?
How experienced are you?
The more experience and riding time, the shorter mast you can ride without ventilating, simply a fact
If less experience, you will think you ride along steadily, while you are actually not