Dmitrij Volkov was injured in a skiing accident in the Swiss Alps. He reached out to clout an unsuspecting child and ended up entangled with the child's sister. They both fell off of a 100 foot cliff. Volkov survived but was confined to a Mechanized Wheelchair due to his injuries.
The scene at the end of the credits for Contract J.A.C.K. hints that John Jack may have done something to Volkov's skiing equipment.