You can get them from sitting for long periods of time too. Anything that reduces the circulation in your legs.
In pregnancy two things are happening, one progesterone is being produced which is a natural relaxing hormone for ligaments, muscles, and yes the walls of the veins.
And we have a massive increase in blood volume during pregnancy. If our veins didn't relax to accomodate the volume we would get very high blood pressure. (BTW this is one reason why some women experience a period of very low blood pressure [sometimes fainting, etc.] right around 17 to 24 weeks as their veins expand and their body is catching up with blood production.)
So, because the walls of our veins are thiner and weaker they can't drive the circulation back to the heart as well, and because there's more volume, the veins get stretched out. As an aside this is the reason why a lot of women get edema (swelling of legs and ankles and feet later in pregnancy). The veins are just not as able to remove the fluid fast enough.
Support hose are your best bet to combat varicose veins (and edema). That helps keep pressure support on the veins and helps them do their job. And when you do get home, get those feet up!
I have terrible awful varicose veins (they are about 40% the ones in the pictures). My poor DH is worried to even touch them as he thinks he'll pop them.
Keep those support hose on ladies!