What causes separation anxiety? Probably any number of things -- anything that can cause the dog to connect being alone with a scary event. As to dealing with it, there are many many ideas out there. You may find a solution that works for your dog; or, you may not.
My dog, Sabastian, is a mixed hound that I adopted when he was 10 months old. His first 10 months are a mystery to everyone. His separation anxiety was severe. I lived in a apartment at the time. I had one advantage--my neighbor like to have "lively" parties on weekends. We had an agreement--I wouldn't complain to the landlord about her parties and she wouldn't complain about Sabastian's screaming.
It was interesting to see how Sabastian and my other dog, Libby, helped each other. Libby, a cattledog, had severe shyness issues. Sabastian was very outgoing. So, Libby was Sabastian's comfort at home, and Sabastian was Libby's ambassador in the big scary world.
After Libby went to Rainbow Bridge last October, Sabastian's separation anxiety exploded again. He became terrible anxiety whenever I left the house--even if my mother was still at home. He seemed to be afraid that I would not return--just as Libby had not come back. Now, whether he really made that connection I don't know but that's what it seemed like. He has begun to settle just a bit now that Tanner has joined the pack.
Some of the things I tried:
Kong toy stuffed with peanut butter: Libby liked these, but Sabastian was too anxiety ridden to give a rip.
Crate training: Sabastian actually chewed his way out of an airline approved plastic travel kennel--pooped plastic for three days. We then got a metal wire crate. He eventually worked several wires loose and I came home to his head sticking out of the crate. He seemed to do much better not being in a crate. Surprisingly, he did not destroy doors, carpet, or furniture when he was loose in the house.
Buster Cube: this was a dice shaped toy that treats went into and when tipped around would release a treat. This was fun for Sabastian but he would only play with it when he had calmed down.
Bach's Rescue Remedy: didn't do a thing.
St. John's Wort: also didn't work.
Valerian: did not work. (I was trying to avoid pharmaceutical anxiety drugs...can you tell? I never would put him on drugs.)
Obedience and trick training: this did help, actually.