Qui-Gon Jinn has finally realized he is an idiot and that everything is his fault. Everything that has happened (fall of the old Jedi order, rise of the empire, the rebellion, the First Order, etc.) can be traced back to his decision to train Anakin Skywalker. If he had just listened to the green troll, none of this would have happened and the galaxy would be at peace. Realizing that he is the retardant-supreme of the series, he has set upon an ambitious plan to correct his past mistakes.
The force is pure energy, which runs through and is made up of all living things. When a person dies, they become one with it; their consciousness being subsumed by its presence. There is no past, present, or future in the force, as time is not truly linear. A strong enough force-ghost who has retained their consciousness can therefore appear and act upon the material world.
Qui-Gon has decided to go back in time to before he made that decision about the brat. Instead of training him after meeting him on Tatooine, he plans to kill him, his remaining family, as well as anyone who traveled with him to the planet. To do that, he must take over and possess the bodies of living victims in order to get there. Unfortunately, he soon realizes that possessing a individual leads to its inevitable decay. The body begins to rot not long after possession, despite the fact that it is still living. This forces him to body-hop from person to person, leaving behind desiccated, withered corpses in an attempt to stay ahead of time.
Why would something like this happen?