It should not be happening at all. Are you at hatch time? Chicks pipping shell with send the humidity way up then it drops. Why hatching at 70% is a good place to be so it doesn't dry out the glue made from those 80% and up spikes. If your incubator is not in the sun then I'd suspect a crummy hygrometer. It's all well and good a hygrometer reads off as you can calibrate it but if it jumps all over the place for no reason then it's useless. Sunlight or other factors causing drastic heat fluctuations is bad but will also drastically effect the humidity- it's relative, i.e. relative humidity (RH).
Also if your incubating, first 18-19 days, at that high of humidity I suggest you lower it. Many like the 30%RH range. But there isn't much you can gauge as to me it sounds like the hygrometer is crud. Get a cheap one and test that one with a salt calibration.
Fill milk/juice cap with salt, add drops of water until saturated. I pour off standing water.
Put hygrometer and cap into sealed container. I use quart ziploc bag and provide small pillow of air when sealing.
Wait 4 hours min, 6 hours preferred and take a reading. How ever off from 75 is your calibration. If your current hygrometer is jumping all over the place in there then it's certainly junk. No matter what it reads it will be steady in salt environment on kitchen counter.
Subtract your # from 75; ex. your reading is 83%, 75-83= -8. You'd always subtract 8 from all reading for true RH.
Write the calibration # on masking tape and stick to incubator as reminder.