The sin of pride, believing you are better than the others. Now many of the problems related to preparing features could be put down to pride, but ultimately the worst of these is pre-defining all the Stories up front because, of course, the Product Managers and Product Owners know best.
Pre-defining all the stories for a Feature or a Release is the classic hang-over of waterfall requirements management practices and order-taking mentality where we: 1) define all the stories up front, 2) implement all of them without any further discussion or negotiation, and finally 3) accept the Feature as done because all of the Stories have been done.
The goal of the team is not to implement all of the stories but to provide a system that enables the benefits of the Feature to be realized, and to do this with as few stories as possible.
Ah, I hear you say, but what if we involve the whole team in the definition of the Stories? This would help us to reduce the time needed for PI Planning and keep us agile. To which we can only say, it might but generally it will take as long outside the PI Planning meeting than it would in it, saving us nothing. Experience tells that it will actually take longer leading to the gold plating of the stories and less agility once we start the real work. We must hold onto our lean principles and create the Stories at the last responsible moment, which, for the initial set of Stories, is the planning event itself.