No one really knows the cause of rosacea but there are many theories. "Rosacea is a skin disease with an obscure and complicated pathogenesis." 
The current most popular theory is that rosacea is a disorder of the innate immune system. For over twenty years the vascular theory was held as the most promising. What is interesting is at the heart of the innate immune system theory is that this includes an overproduction of cathelicidin (a killer of microscopic organisms that is at the cellular level found in white blood cells and also found in cells on the skin) which is transported in the vascular system, so we have come full circle. Some consider the nervous system as the root cause of rosacea, and the list goes on with many other theories worth considering (the latest theory is the Trigeminal sensory malfunction theory) but a good place to start your research is our post on Theories Revisited.
Rosacea and Helicobacter pylori: links and risks
Elizabeth Lazaridou, Chrysovalantis Korfitis, Christina Kemanetzi, Elena Sotiriou, Zoe Apalla, Efstratios Vakirlis, Christina Fotiadou, Aimilios Lallas, Demetrios Ioannides