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.