I am standing in my cubicle at work now, doing a low side kick in an attempt to recreate the loss so I can explain it to you, and of course I am not losing my balance!
Now that I think about it, I think the loss occurs only when I go higher. For example, there is a section in the wooden dummy when we step back from it and deliver a side kick that lands just under the lower arm, as if we were kicking someone in the torso. During this motion, I usually find myself falling to the side during the kick. Which side depends on the leg that is kicking: I fall in the direction of whichever leg is still on the ground.
Another phenomenon is that, when I bring my leg down after the kick, I find myself "falling" into it, if that makes sense. In other words, instead of controlling my balance so that I bring my leg back to the ground in a steady stance, I pitch forward. This could lead to something very dangerous happening...for example, if I pitch forward into a punch.