My dog only snaps at other dogs when shes in the crate sometimes, why?

It sounds like she is just protecting her space, or feels trapped. Either way you dont want it to continue. But why is she in the crate locked up while other dogs are roaming? Try correcting just by saying No when she does this. You will have to pay more attention to whats going on to catch it as much as possible so you dont confuse her by only saying no sometimes.