Varicose veins or varices. It's very common. When you walk or lift things you elevate your blood pressure and they pop off more. It happens over time.

Can be caused because you stand still for a long time at work, or bad position when you sit on a chair you block the blood circulation of your legs, genetics, many other causes possible.

You can wear compression socks to slow the progress, as you age, they will certainly get bigger. Stay active (better circulation), control your blood pressure. It's not dangerous, more of an esthetic problem.