Rosacea is a long-term skin condition that typically affects the face. It results in redness, pimples, swelling, and small and superficial dilated blood vessels. Often the nose, cheeks, forehead, and chin are most involved. A red enlarged nose may occur in severe disease, a condition known as rhinophyma.
The cause of rosacea is unknown. Risk factors are believed to include a family history of the condition. Factors that may potentially worsen the condition include heat, exercise, sunlight, cold, spicy food, alcohol, menopause, psychological stress, or steroid cream on the face."

Source :Wikipedia

As well as having unknown causes, the incidence of the disease is also unclear - estimated at between 0.5% and 10% (note the 20x margin of error).

As described above, it's known that a variety of factors - or combination of factors - can trigger episodes, but the bio-chemical mechanisms which lead to the condition are unknown.

For extensive details, see Pharmacy and Therapeutics (34)1.