Hamish Denham, the study's lead author, says that even the highly disciplined police dogs appeared to lose it when kept confined, exhibiting manic behaviors associated with failing mental health. The dogs were recorded repetitively jumping and bouncing off walls, pacing back and forth, spinning, and walking in circles around the perimeter of their pens.
The study notes that "some dogs may find isolation from humans particularly aversive, hence affecting their reactions both to being left in a kennel and to being taken to the veterinary surgeon."
Researchers provide no definitive explanation of this behavior, but it seems rather intuitive. Extensive studies on the effects of solitary confinement of humans has shown similar symptoms of mental disorders -- with many considering the punishment to be a form of psychological torture.
While this study focuses on dogs that are properly stimulated throughout the day, the effects of isolation for the thousands of dogs kept in rescue center kennels is likely much more profound. A paper released by the Stafford Animal Shelter in Montana describes a mental illness, known as "kennel crazy" or "kennelitis" in canines left too long in confinement: