It helps to understand the circulatory system to understand AVMs.
Oxygenated blood flows at relatively high pressure through our bodies in arteries.
Deoxygenated blood then flows into veins and back to our hearts. The veins, as a result, are forced to carry a higher pressure load than they are designed to, and after years of such duress, can rupture. These AVMs aren't as dangerous as a standard AVM because of the relative lack of blood pressure, but can still result in serious neurological repercussions. I'm no doctor, but that's what they told me. There's tons of info on the web, if you search for it.