As TFA says, they were specifically looking for the effect. It an "HDAC inhibitor." These uh, inhibit the enzyme HDAC, which is known to shut down brain plasticity in adult (mice). As to why they figured that out, some of the studies cited in TFA were looking at brain plasticity, plaques, brain damage (strokes?) and (of course) Alzheimer's.

BTW, one of Sodium Valproate's trade names is Depakote. Very commonly prescribed (or at least used to be) for treatment of bipolar disorder and epilepsy.