Polyurethane "separating" on table top
A couple possibilities come to mind:


The product was not mixed well
The product has exceeded its viable shelf life
The product is contaminated
The table surface contains contamination
The table is made from a very oily wood that the varnish is not bonding well too
Applying it with the wrong applicator
Other application issues - wrong solvent used to thin it, applied too thick, etc.


I would start by ruling out the varnish as the issue by trying another can, manufacture, etc.  

If it turns out the wood is the problem, you might also try applying a thin layer of a dewaxed shellac as a sanding sealer first.  More difficult woods might require something like Clear Penetrating Epoxy Sealer or a Hardwood Epoxy Glue as a base layer to the varnish.

I have no affiliation with these products and only recommend them based on experience