Are your sitter's trained to observe "pack" behaviors when sitting multiple dogs in a home?
Last time I had my dog boarded by Rover, she returned in a fearful state. I'm guessing she was picked on by a more dominant dog that was in the home. Is this something that sitter's are expected to observe and act on? If I use Rover again.....I'll be sure to choose a different sitter.
Some dogs exhibit a fearful state when apart from their owners. And fearful dogs sometimes behave aggressively. If so, they are treated as a watch. So maybe your dog was not "picked on."