Russian troops have always been in the Crimea, certainly not as much but they were.
Crimea is not important for what reasons do not support the new government, the new government needed Crimea. Suppose Russia did not intervene, then the common people of the Crimea with hunting rifles will fight with maydansbrotherhood with firearms. Many corpses - less taxes, Russia risks losing its investment and fleet. (china way, too), because the new Government may refuse to pay debts and fulfill agreements with criminal president.