Astronomers are still stumped because of the odd tilt that our Solar System has. We can't seem to find any logical explanation for it, although one thing that keeps being tossed around in the scientific community like a hot potato is the theory of Planet Nine.
For those unaware, Planet Nine is basically a huge planet floating around somewhere in the distant Solar System and it has yet to actually be discovered. Nevertheless, it's believed that Planet Nine has a huge gravitational impact on our Solar System and could be tweaking things around a bit.
The solar system is tilted by up to 6º; our planets are all rotating around the Sun on wonky planes, and that didn't just happen by itself. Something must be tugging on everything.
With everything we've observed thus far about the Solar System, such as smaller objects in the outer solar system orbiting the sun in a very odd way and the weird tilt we see, computer simulations have pretty much all but confirmed the existence of Planet Nine because the numbers add up with predictions of the planet's existence.
We're still just trying to find it and snap a photo of it. This could take a while because the observation area is so large, but within just a few more years, a discovery may be possible.