What is the accepted theory as to why Uranus' axis is tilted so severely?

To add to the existing answers and adapting from my answer to this question about the Uranian satellites:

The single giant impact idea appears to be unlikely. In this scenario, the circumplanetary disc would re-align with the Uranian equator after the collision, but would end up with a retrograde rotation and thus produce retrograde satellites rather than the observed prograde system (Morbidelli et al. 2012).

As noted by Rogoszinski & Hamilton (2020), the timescale for tilting Uranus via a resonances between the precession of the rotation axis and the orbits of the other giant planets appears to be too long compared to the expected timescale for giant planet migration in the Nice model: they estimate ~10⁸ years to reach a tilt of 90°. Under ideal conditions, they estimate a tilt of ~40° could be reached in ~10⁷ years.

From the abstract:

We find that two collisions totaling to 1 M_⊕ is sufficient to explain the planet's current spin state. Finally, we investigate hybrid models and show that resonances must produce a tilt of ∼40° for any noticeable improvements to the collision model.

But as noted in usernumber's answer, this is an active area of research and our knowledge of the ice giant planets is woefully incomplete.

There is not a single accepted theory as to what caused Uranus' tilt. Rather there are two families of explanations and we still need to collect more data to figure out which is correct.

As explained by @Timtech, Uranus' tilt might have been caused by a giant impact. This is supported by simulations showing that an impact might have been violent enough to tilt the planet and eject debris that agglomerated into moons. But this is not the only possibility.

Another explanation for Uranus' tilt is a spin-orbit resonance. As Uranus was a growing protoplanet, its disc generated a resonance, causing the disc in which it was forming to tilt.

Simulations have been done showing that the tilt might have happened during planetary migration without any giant impacts.

Three of the linked articles above are from 2020, showing that this is still a hot topic of research and there is no consensus on what happened to Uranus.

Nasa.gov speculates that the most likely possibility is that an object with a mass close to that of Earth's collided with Uranus, causing it to rotate on its side from then on.

A recent test by Space.com suggests that "Planet Uranus Got Sideways Tilt From Multiple Impacts". These findings suggest that two or more smaller collisions probably occurred, asserting that the early solar system was probably more turbulent than most would think.