Why do some people have bulging veins in their foreheads?

Usually: Born that way. Quite frequent presentation. Certainly, body builders, weight lifters can develop veins on the forehead, i suppose. Maybe a trumpetist as well...

Multiple reasons.: Facial veins like hand veins often change in size throughout the day and is due to many factors such as hydration, sun exposure, hereditary, heat, physical activity, stress and anything that would make the veins dilate or constrict. There are also some medical conditions such as heart failure, kidney and liver failure which could contribute but not in a healthy 34 Y.O.