I don't have a water-lock crock, but I sort of wish I did, just to play with it.
I'd be interested, if someone who has one empty was willing to fill the moat of a lidded empty crock and then hit the bottom of it with a hair dryer and report back how much bubbling and how responsive it seemed to be.
The
Ideal Gas Law tells us that if the amount of gas is constant (which we can't count on, in fermentation, but it has to be pretty close over short periods of time), then pressure times volume = temperature. So when the gas inside the crock cools, PV decreases as the gas contracts, drawing water in the moat toward the inside. If it cools enough, either water enters the crock proper or bubbles from outside transit through the water and into the crock. The latter is what I assumed would happen, but I suppose it depends on how full the moat is..?
We know for sure that normal lacto-fermentation (and also fungi eating sugar) gives off CO2. And in my experiences with lots of veggies, but no moat-style crocks, when I have sealed jar, pressure always builds up over time and negative pressure never happens. But, y'know, there's lots of room for not knowing stuff.
Maybe one of the processes that our microorganisms engage in is fixing atmospheric gasses into solids in the brine. They might well do that some during reproduction, for instance. As the mass of the colony increases, that matter has to come from somewhere. It could be coming from your produce, but it could also be from the gasses above the brine. Maybe some bugs do that more than others. Or maybe, during some stages of fermentation pipeline, that happens more than during others.