Why do veins undergo vasoconstriction during physical activity?
I believe that the answer is in the same cited source (BioSBCC).

Veins are considered a blood reservoir and undergo constriction to mobilize blood.


  [...] the veins act somewhat like a blood reservoir, containing 60% of the total blood volume at rest. [...] When the body needs to mobilize more blood for physical activity, the sympathetic nervous system induces vasoconstriction of veins.


The result is an increase in circulating blood volume. This changes cardiac output and arterial blood pressure (CV Physiology: Blood Volume).

I also recommend reading on Biology.SE: Where does extra blood come from to fill your muscles during exercise?