If I'm not mistaken, one reason for vascularity like this is a transition from being severely overweight (fat). Veins actually trail through your fat when you get hefty enough. When you lean out, the veins do not go away.... and there's more of them.I'm not talking about visible vericose veins either.
A lot of former fat guys are very very very veiny lean. Seen it first hand, huge ab veins and shit on one of my buddies who used to be like 80lbs of fat overweight. Others just have the genetics.