Actually Uranus has an axial tilt of 98 degrees, virtually having its axis pointed at the Sun, so it is really a bit arbitrary to say whether the original North Pole is the one pointing out or pointing in, so the interpretation that it spins opposite to the other planets is just a wee bit interpretational.
The clockwise spin of the planets is mainly due to the clockwise rotation of the proto-system. Once the cosmic dust started to rotate as the Sun coalesced, it set up the conditions to cause the planets to rotate clockwise as they coalesced as well. Since the dust inside the orbital track of the planetary node would be moving slightly slower than the dust on the outside of the orbital track, it would set up a tiny accretional and frictional bias (With a large cumulative effect) in favor of clockwise rotation of each forming planetary body.
Captured bodies originating outside the system, evolutions of pole orientation by planets that do not have a large stabilizing satellite, or major collision after-effects can produce counter-rotating bodies, but they wouldn't naturally form in place that way.