When dogs are confused they will revert to what they know best or learned last. When training a dog to whoa that has had sit first will invariably lead to a dog that will sit when being commanded on whoa. It is just another thing to deal with . I like to make whoa bomb proof first ,that way when the dog is learning any new behavior the worst thing that will happen is you have a dog that is standing still.A dog that sits under pressure is sitting because it is confused about the command and is being prematurely pressured. A dog should be 80% positively responding to a command before any pressure is used. If a dog is sitting on the whoa command it is offering a behavior because it believes that is the behavior that will get the reward, this is not a time for correction ,it is a time to teach the command. A simple eh eh will suffice at this time , then show the dog what you want.