An vehicle falling into a pit would be considered collision. The vehicle would definitely collide with something when it fell. 

 Here is an easy way to remember collision vs. other than collision:

Collision - the vehicle is expected to be under the control of the driver (There is upset or impact with another object.)

Other than Collision - the vehicle is not expected to be under the control of the driver (theft, fire, earthquake, explosion, etc.)

I hope this helps.

Jan