Over sanded between coats of varnish
Wow that is still too coarse unless you had bubbles. I actually use steel wool or 400+ grit. With 220 and even light pressure you cut through the layer, the second you use anything to scuff the surface it will look dull or even white this is normal clean and apply another coat, when you sand between coats you are taking off the high points and roughing it up so the next layer will bond better. On high end projects that I want an absolute glass surface that looks to have depth I will use 400 grit until the grain no longer shows, then I jump to fine steel wool or 600+ until I get the depth or look of depth I want. When you add the next layer if well cleaned it will become clear and look great again. 

One trick to save your brush, I use a glass jar that can be sealed, fill with solvent to cover the bristles, immediately after finishing I clean most of the varnish out (you never get it all out) I put the brush(s) in the jar and seal it I have had quality brushes last 6+ years using this method, time for a second coat or 10th, pull the brush out shake off the excess load the brush and go. Cleaning at the end and returning to the sealed jar. 
I use +20$ brushes and prior to doing this I could waste 1/2 a gallon of solvent and the next day the brushes were ruined as the fine bristles were hard and sticking together, so a nice coat was impossible. Using this method I have had to replace some smaller brushes that fell apart after many uses but the big ones have lasted all these years and work as well as a new brush.