Found a few things on the internet about a lawyer withdrawing from a case...
"However, abandonment may be acceptable even if it harms the client's interests, especially if the client has done something wrong. For example, a lawyer can walk away if the client is engaged in a continuing criminal enterprise, if he's using the lawyer to perpetuate his illegal scheme, or if the client asks the lawyer to do something illegal himself. Deadbeat clients also risk abandonment, as do those who refuse to cooperate in their own representation."
Then "Criminal defendants have a constitutional right to take the stand in their own defense. Occasionally, one of them tells his lawyer in advance that his entire line of testimony will be lies. This scenario presents what legal scholar Monroe Freedman famously referred to as the lawyer's "trilemma." The attorney has an obligation to fight for the client's interests, a responsibility to identify perjury to the court, and a duty to keep his client's secrets. Because the client has put the attorney in a situation in which it's impossible to fulfill all three professional obligations, some lawyers see this as a situation that demands withdrawal. Unfortunately, it's not that easy. As mentioned above, an attorney can't withdraw in the middle of litigation without the judge's permission, and it's indisputably unethical for an advocate to directly inform the judge that his client is a liar. What usually happens in these cases is that the lawyer approaches the bench and asks to beg off the case for vague "ethical reasons." The judge, knowing exactly what's going on, typically denies the request, because the jury would smell a rat if the lawyer were to disappear right before the defendant took the stand."
-Source:
http://www.slate.com/articles/news_and_politics/explainer/2011/04/pulling_out.html
Also, much more prosaically, Wikipedia says a lawyer can *ask* if the financial burden on the lawyer is too great, or the client refuses to follow the advice of counsel, or
engages in acts relating to the representation without informing the attorney or seeking the attorney's advice. (BBM).
I find that last one intriguing in the light of the subpoena. Could it be they know/suspect she's been in phone contact with someone and trying to get them to perjure on her behalf in the penalty phase or otherwise engage in something illegal? Hmmm.
Another web site put it a different way "..the client insists upon pursuing an objective that the lawyer considers repugnant or imprudent". Although I can't imagine anything KN would find repugnant..... (snark).