One cause would be that if you're thrown off balance, you don't have a stable position and can't execute taijutsu moves properly.
Also, if I try to imagine how I would react when I was thrown off balance, it would force me to focus on not falling to the ground, since that would give my opponent the upper hand. Allthough most of my movements would be reflexes, I would be too distracted to focus on my opponent. I would also let my guard down, what makes me vulnerable.
So, all in all, throwing an opponent off balance weakens his defense as well as his offense.