It causes the humidity & temperature to fluctuate. When you are on the last 3 days of hatching, the humidity changes can cause the membranes of the eggs to dry out, and then the chicks can't hatch. Been there, done that.
Consistent temp and humidity are critical throughout incubation, but of utmost importance during hatch time. Opening the incubator causes the entire environment within the incubator to change, thus potentially causing problems (regardless of how fast it "gets back to normal"). Quail are really sensitive to these changes and need the constant humidity for their expedient hatches. They are slow to start, but typically when they start they pop like popcorn. Opening the incubator can force the membrane to shrink wrap around them and make the whole process shut down. I always stress keeping it closed to ensure good hatches. I've been there and learned from experience. I hatch thousands of eggs a year and learned the hard way how important this is. Good luck with your next hatch.
I incubate in styrofoam incubators and hatch in a plastic Brower Top Hatch Incubator. I leave my chicks in there for 2 or 3 days until all have hatched that have pipped. If there are unpipped eggs 2 days after the first one hatched I shut it down. Chances are they won't hatch and if they do they may pass that characteristic of late hatching on to future generations. Remember that they can hang out in there for up to 3 days with no food and water. Its hard to watch, but it is best for all of them to get a good start and a chance to hatch.
I'd really like to open it to at least remove the little guy that was hatched on Wednesday. I don't know what's going on with these eggs (purchased different places, but set the same day) Banty eggs take forever to hatch..
Anyway, I'd hate for him to starve- it's been two days.
The yolk that was absorbed before the chick hatched will feed him for about 2 days. So relax, he won't starve!!
Oh btw, it must be nice to have late hatching banties!! Mine have hatched at least 2 days early every time so far!! They are usually still in the turner when they start pipping. Even my hens hatch 2 days early!