We think Uranus was hit by a real big 'planetessimal' soon after the solar system was formed, but before its satellites were born. Perhaps the impactor put matter in orbit around the new tilted Uranus so that the satellites could eventually form out of this ejected material left over from the collision. It seems that all of the planets have at one time been hit by BIG OBJECTS nearly as big as they are. For the Earth, this resulted in the formation of the Moon.
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