What conditions could cause major leg cramps?

Several things. Tired muscles from heavy exercise, prolonged standing/working on them. Abnormalities in potassium, calcium, magnesium, dehydration, venous disease (venous insufficiency), poor arterial circulation can result in rest pain which some describe as cramping.
Many things. As dr. Isobe rightly said, there are many things that can cause leg cramps. However, venous insufficiency is a more common cause of leg cramps that calcium, magnesium, iron, and potassium defiencies combined.