First of all, Zimmerman’s attorneys can’t withdraw from the case, because as of the present, there is no case. Zimmerman has not yet been charged with anything.
Zimmerman apparently couldn’t take sitting around in silence, and has decided to be more proactive in presenting his side of the story. Of course the standard advice from defense attorneys is to keep your damn mouth shut. But I’m not sure it’s good advice in this case, as the “case” is being tried in the media rather than in the courtroom. Anything Zimmerman says can be used against him if he is ever tried in court, but maybe that doesn’t matter if his story is consistent and he’s truly innocent. Lawyers are so used to dealing with guilty clients that they don’t know what to do when they have the extremely rare innocent client.
The fact that the two lawyers were working the case pro bono may have contributed to their eagerness to withdraw. Also, I previously pointed out that Sonner is a DUI attorney who has no experience with anything like a murder case. Zimmerman needs some private eyes who will dig up dirt on Martin (like how OJ's private eyes dug up dirt on Mark Fuhrman, although the same private eyes were unable to find the "real killers").
Zimmerman calling Angela Corey is an interesting move. He probably tried to plead his side of the story to her, but I'm not sure if she's a person with normal human emotions who would have any sympathy for him.