This argument against the notion of PAS is basically a chicken-or-the-egg argument. The thrust of the argument is that women who go to get abortions are already more likely to suffer from mental illness. Therefore, it is not fair to draw the conclusion that abortion causes higher depression rates; any correlation between abortion and depression may simply be explained by the fact that more depressed women go to get abortions.
There are many possible explanations for why women that seek abortions might already have a greater propensity to have mental disorders. First, sexual promiscuity and unprotected sex (both leading causes of unwanted pregnancies) are often associated with risk taking and careless behavior. It can be argued that it is more likely that careless risk-takers suffer or will suffer from mental disorders. Again, the causal link being drawn here is that existing behaviors and mental disorders more frequently lead to the need for an abortion. This is opposed to a causal link that leads from abortion to mental disorder.