Dandelion and Citris juices act upon the digestive system. Stirs things up. That can cause gas.

Sometimes the gas is because you're cleansing, sometimes it's because the veggie you're juicing is a gaseous veggie for you. Some veggies are gas producing for certain body types, body types as in Dosha, from Ayurvedic medicine, which very accurately predicted which veggies would be gaseous for me.

I'm a Vata Dosha, which is naturally airy anyway, so it's easy for me to develop gas. Pitta dosha is heavier and usually gets constipated rather than gas. The list of things that I'm supposed to avoid includes Broccoli, and sure enough, anytime I eat it, especially when raw, I get terrible gas. So I limit it and only eat it cooked.

If you don't know your dosha, click on the link and find out! It's really very fun. :)

Dosha Test