The most common cause of a leg ulcer is a faulty vein(s) that allows blood to leak or travel the wrong way in your lower extremity. A useful analogy to how of these leg ulcers form is to think of your vein as a leaking pipe and your skin is dry wall. Now imagine the leaking pipe is causing the dry wall to get wet and eventually rot. The damaged dry wall will crumble and maybe even develop an open hole. This open hole is the venous ulcer.