Seems I was forgetting that the initial collision does
not create the anti atoms, The Momentum
is conserved as the collision makes the pair involved in the collision get extremely hot. The Anti matter is then a by product of this extreme heat.
The Law of the conservation of Momentum states that for every action there will always be an equal and opposite reaction. In the above example the thrust of the particle being fired from the particle accelerator is equal to the thrust of the the target being forced backwards from the recoil of the impact.
Wiki Momentum