How did mosquitoes expand their habitations to a place they never lived before?
It could be a number of things. Mosquitos need standing water. Look for that. Any changes? Was last year particularly rainy?  One group of mosquitos lays its eggs at the waters edge. They lie dormant until the water reaches them again. If the water was high when the eggs were laid, then recedes, it may be years before the water reaches them again and the eggs hatch, resulting in a bloom of high numbers. Also, other mosquitos breed in containers. Things like tires, or coffee cans, or flower pots in cemeteries. Even a small amount of stagnant water in a container can result in mosquitos. Any changes there? The obvious change is the warming climate, but I would look first to changes in the water.