Collision means the impact of your insured car with another object or upset of your insured car. Loss caused by missles, falling objects, fire, theft or larceny, explosion, earthquake, windstorm, hail, water, flood, malicious mischief or vandalism, riot or civil commotion, colliding with a bird or animal, or breakage of glass are other than collision losses (a.k.a. comprehensive).
You will need to ask your insurance company to clarify why they believe this should be considered "collision." But, they may be classifying it as an upset of your insured vehicle.