Mecanim additive animation sends characters flying?

Through testing we have found that if any of our animated characters has a downwards or upwards velocity and we then play an additive animation on the character, he will go flying away.

Initially this happened whenever the animation played and I fixed it by setting a body mask on the animation clip with all the IK removed.

We are still experiencing this if the animation plays while a character is falling for example they will skyrocket off the map.

Any ideas what is going on?