The authors make a good case that cascara, as used in the studies, increases nitric oxide which is then involved in causing diarrhea. They also showed that in a prior study were they inhibited NO-synthase activity with L-NAME or stimulated NO production with L-arginine which reduced or increased cascara diarrhea respectively (http://www.ncbi.nlm.nih.gov/pubmed/8773457).
However, the dose used in both studies (800mg/kg cascara) was excessive, for a 70kg human it would equal 56g of cascara. The usual recommended dose are in the range of 250-2000mg, which would equal 3.5-28mg/kg body weight.
So I think from those studies we cant make any claims on the mechanism of action of cascara at the much lower doses as widely recommended. For this we would need an adequate trial with lower doses in humans.
In theory though, I find it likely that significantly increases gut motility or diarrhea must have some kind of irritating property through serotonin or other mediators. Its the physiologic function of the gut to retain as much water and nutrients as possible for proper nourishment of the body, and any emergency gut evacuation mechanism probably must involve some kind of stress response (i.e. get rid of dangerous bacteria and toxins).