1. physiologically speaking, the blood vessels in arms and hands are restricted and the blood flow applies more pressure to the vessels, which makes the vein bulge out. [because they are closer to the skin].
2. It may be due to venous insufficiency
. i.e. not able to push blood upwards and back to the heart.
i. Bulging arm veins should not be confused with varicose veins
which is a seperate condition.
ii. Veins expand when HOT causing easier blood flow [occasional episode in your history]. You should see a doctor for any vascular disease
[if still anxious]