The exact history of each planet’s axil tilt is a mystery, but most of them seem to have been rocked off center by gravitational interactions and massive collisions with moons and proto-planets that are now gone.
We only see clean orbits and stable moons today because we’ve had billions of years for crossing orbits to collide and unstable orbits to decay.
Some of these impacts were truly cataclysmic. Earth’s own moon is thought to be debris from a planet the size of Mars striking and liquefying the Earth, and Saturn’s moon Mimas has a crater so large that the impact must have nearly obliterated it.
It’s hard to maintain a perfectly perpendicular rotation when another planet comes streaking in off-axis and launches half the mantle into orbit.
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