What would make a female dog have a false pregnancy?

If a dog already had puppies, would she have a false pregnancy the next time?

False (or phantom) pregnancies are fairly random events.

Some b*tches may never have them, some may have multiple ones, it's all down to the individual dog.
I have seen some b*tches have phantoms even though they have never had a litter. Some have pups at 2 years of age, never have another litter, but will then have a phantom in their "elderly" years.

Are you positive what your dog is experiencing is a false pregnancy? There's no way a male could have got to her during her heat?

Phantoms can be very confusing for the dog and the owner.
Some b*tches have very convincing symptoms, they can gain weight, even produce milk (and thus contract mastitis). They often adopt toys to serve as puppies and get distressed if the toys are removed.

Most B!tches go through two heat cycles per year, sometimes if she has not mated or been mated with an infertile male she can get/become pseudo or phantom pregnant. The best thing you can do for you pup if she is not breeding quality is to spay her. Phantoms can be very distressful on your dog, especially if she is looking for her puppies. Her body is giving her all of the signs that she should be whelping, yet there are no babies to suckle. This situation will not only create a psychological toll, but also a physical one that will cause confusion and stress to a female dog that is dealing with a false pregnancy.