Over time, some of those smaller objects passed close to the gas giants. Those close encounters caused the orbits of the gas giants to expand, albeit very slowly. Jupiter’s orbit actually got a bit smaller. At some point, Jupiter and Saturn’s orbits got into a resonance, causing Jupiter to go around the Sun twice every time Saturn went around once. That caused havoc.
A lot happened very quickly, by solar system standards. The almost-circular orbits of Jupiter and Saturn got stretched, and Saturn, Uranus, and Neptune had several close encounters.
It needed modification. One suggested change was to add a fifth gas giant. In simulations, the event that triggers the LHB also ejects the hypothetical fifth gas giant from the Solar System. Such simulations lead to a solar system that looks like the current one, so it’s not an unreasonable idea.