Blushing in shellac I've only seen a few times and it was always when it was sprayed.
As Chuck said, it's trapping moisture under a skin that is formed that causes it. My expertise is in spray-painting in the automotive trade. Been at it for over 30 years now. But we don't use a lot of shellac there.
I very well could stand to be corrected here, but one source of water / moisture in shellac is the alcohol itself. A lot of alcohol that you get is not 100% pure, and even those that started out that way will absorb water if left open. Thinking of the cheap hardware store variety that we get here in Australia. Some is so bad that shellac won't even dissolve in it.
The next area of possible moisture contamination comes from your compressor. It's the nature of the beast, and you need to have a good water / oil separator installed at the very minimum of 3 meters down line from the compressor or you're just waisting your time. Then finally there is the humidity that is in the air when you are spraying. This is a big one for most people, as the other ones can usually be dealt with. The only thing that you can do is wait for better conditions.
When it comes to spraying shellac, I don't know of a lot of things that you can do to stop blushing other than using 100% alcohol. You can't use any sort of retarder that I'm aware of, as they aren't compatible with shellac. And spray when conditions are optimum.
Better yet, just wipe it on with a rag in very thin coats and you should be good.
One good thing about blushing in both shellac and lacquer though is that they are easily repaired. For the shellac, you can come back with another thin coat when conditions are better and all should be good. I've also just did a wipe over with some shellac on a rag, over the affected areas and they disappear.
With lacquer always add extra slow thinner (retarder) to the mix. Never more than 10%. This will keep the surface open, help with flow out, and you'll use a lot less material to boot.