The most popular theory for Uranus's extreme 98-degree axial tilt is that it was struck by one or more massive objects, likely Earth-sized protoplanets, early in the solar system's formation roughly 4 billion years ago. This "giant impact" hypothesis suggests that such a cataclysmic collision provided enough force to knock the planet onto its side permanently. While a single massive impact was the long-standing primary explanation, modern computer simulations often favor a "one-two punch" of multiple smaller impacts to better account for the fact that Uranus’s moons and rings also orbit on this tilted plane. A prominent alternative theory gaining traction involves the gravitational pull of a large, long-lost moon that may have gradually "tugged" the planet over millions of years through orbital resonance before eventually crashing into it.
