A motion to withdraw is when an attorney is asking the court to allow him/her to no longer represent the client. Generally because of a conflict of interest, or because of irreconcileable differences between the client and the attorney.
Attorney's take an oath to represent all of their clients to their full capacities. In this instance, there has been an event of some sort that will result in the attorney not being able to represent the client's case in their full capacity.
Keep in mind, I am not an attorney. You would probably be better off seeking the advice from another attorney in regards to this.