If A says something surprising or shocking to B, B may not know what to say or how to react for a short time. It's like B is not able to be stabled and balanced because A's words have destabilised him or her. A has thrown B off-balance.

"sb. throw sb. off balance" > to say or do something that caught the other person by surprise , and it made him temporary not certain how to reply or react to it > so he is off balanced (for a while)