Therefore, what is weird/rare about that photo to me - is that a person who had suffered childhood trauma by specifically being zipped into a suitcase (a memory strong enough to mention and write about as the specific abuse they suffered) would find it funny and enjoyable to recreate that experience. It is of course possible the situation left no lasting effect on him, but then it is strange that he specifically talks about it.
The simpler explanation is that everything Rob Young has written or said about his childhood is completely made up. It was all just a bunch of lies to sell some books and make money from sponsorship. You don't need to be a doctor to see that.