Do they have to give you a reason or can they simply withdraw?
Landlord / Tenant Lawyer
Normally, lawyers give their clients reasons for leaving cases. You should look at your contract with the lawyer. It may give you some clues on why he withdrew. Also, a lawyer can withdraw for ethical reasons. If you have further questions about the matter, write your attorney for an explanation.
Otherwise, look at the Georgia Bar's Consumer Assistance site to learn about possible steps. http://www.gabar.org/programs/consumer_assistance_program/information_for_consumers/
If you agree to substitute them out, it doesn't matter what the reason is, you have the absolute right to fire them at any time.
If you dont agree, they can ask the court to withdraw if they MUST withdraw because of somehting unethical you've done or will do that puts the lawyer in a bad spot. Or they can ask the court for permission to them withdraw, if your rights won't be prejudiced. The most common reasons in motions to withdraw are breakdowns in communications and failure to cooperate, which are nice ways of saying you breached your agreement to pay your legal bill and the lawyer doesn't have to work for free. These motions are routinely granted for those reasons.
Disclaimer: I'm only licensed in CA. Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
Estate Planning Attorney
Pamela answered it very well. In California, if you don't agree, I ask the court to allow it. As she points out, usually is the break down of the relationship because client failed to pay as agreed [none of wants to work for free] or because the client is going to do something illegal, like lie on the stand. I cannot tell the court you are going to lie on the stand, for example, but I must move to withdraw as your attorney. Not knowing YOUR facts, I am just giving you examples here in California.