Why would someone have to surrender their law license?

They probably did something unethical and the Bar took their license. Without more info one can only speculate on the exact reason. Suffice to say, it takes something really bad for that to happen.

Obama lied on his liscence application, MO had to drop her law liscense by court order.

Without more information there is no way to really answer that question.

Sleeping with a client is a good way.

Addiction is another.

But just like ac said, you have not provided us with sufficient information to give you a good answer.

Abuse of the law any which way you look at it!

There are any number of disciplinary causes to censure, limit, or disbar attorneys.  Any willful breach of ethics or failure to work professionally is suitable cause to move the Bar Association to act.