This isn't unusual donkey behavior! Donkeys are very tactile, affectionate animals among themselves and their human companions. They are fond of doing what I call "wrapping" where they cling to you and wrap themselves around you. Unlike some horses, when a docile and trained donkeys puts its butt next to you, it's because they are comfortable next to you. They may be signaling that they want their rear end scratched, or they are standing as they would next to another donkey, side by side. Donkeys are also protective of their pals, and maybe assuming the "watchdog" position, much as a dog may sleep at the end of their owner's bed, facing the door, to instinctively protect against intruders.
Two of my jennies like to parallel park on either side of me and gently squish me between them. They're just seeking affection, and if I move back, they'll back up with me.