My pet theory (which is not intended to be taken too seriously) is that it is indeed a discarded light sail, and that light sail in particular was used to transport a Von Neumann probe which detached at some point after the object entered our solar system but before we first observed it. This detachment, in addition to leaving the sail tumbling, would have changed its trajectory, meaning our estimation of which direc…
Are you sure it's a coincidence? Imagine you are an intergalactic species that advances yourself by invading other worlds and stealing the inhabitants' knowledge and technology. What would be the best time to invade? I would suggest waiting as long as possible in their technological advancement but just before they develop the ability to defend themselves.