"One of the great mysteries of our Solar System is why Uranus is tilted on its side. Surely, if the solar system formed from the same rotating cloud of dust and gas, then all the bodies within it should rotate in the same way. And yet Uranus' axis of rotation lies at 97 degrees to the plane of the solar system.
The standard explanation is that Uranus must have been involved in some kind of interplanetary collision with and earth-sized protoplanet in the early days of the solar system. That's a tempting idea but it has some shortcomings. For example, it doesn't explain why the orbits of the moons of Uranus are similarly tilted, not that of its rings.
Today, Gwenael Boue and Jacques Laskar at the Observatoire de Paris in France put forward another idea. [..]"
arXiv blog
added later: After reading the paper I have to confess that I still don't really understand how the trick works. I would have thought one needs to assume a multipole moment for the planet (Uranus is not a perfect sphere) and/or perhaps some tidal effects, but I don't really see a discussion of that in the paper. Of course, there are a lot of references to previous work...