What causes veins to pop out on your forehead?

A lot of things.: Increased "valsalva" from laughing, straining, etc., exertion in general, thin skin, heat, pale skin making them more visible...lots of things. There are virtually no serious medical conditions that would cause prominent forehead veins and those that might would almost certainly have other significant symptoms. A vein specialist can often eliminate these veins if you'd like.

Positional change.: The veins in the face just as the veins in the rest of the body change in size depending on changes in position as well as changes in levels of hydration and changes in pressure in the chest and abdominal cavity. In the face, the veins decrease with standing and increase with lying down. Straining or laughing increase the the pressure in the chest which can increase the size of the facial veins.