There are a few reasons varnish can leave bubbles behind.
The sealer coats, or first couple layers of varnish that are applied directly to the wood, may get bubbles as the varnish penetrates the wood, displacing air from it. Once the wood is thoroughly sealed, this should stop occurring.
Another reason bubbles happen is that the wrong type of brush was used to apply the varnish.
in later coats, brushing technique is the biggest cause of bubbles. Working too quickly or whipping the brush around can also create bubbles. Adding a touch of brushing reducer can sometimes relieve these bubbles before the varnish cures, but it may take some trial and error to figure out the minimal amount of reducer to retain the best finish, without bubbles.