I agree with Rob: Dust. The 100 grit sanding will also leave relatively deep scratches which will trap dust. If you just don't feel like sanding any more (maybe you are starting to hit the glass) you can varnish it as is (you probably already did) and then sand with finer grit (220) before the next coat. Dust and bubbles only really count on the last coat. They can be sanded off until then. Use a tack cloth to help remove dust. If you are in a dusty environment, tack cloth immediately before varnishing each section. While this will not stop more dust from falling on it, it will help reduce the little bubbles.