Musk borrows against his Tesla shares to fund his investments into SpaceX and SpaceX owns Tesla bonds. The two are pretty significantly linked financially. If Tesla has issues with paying back bonds, it would probably impact SpaceX.

Additionally - I don't know where price-wise Musk gets margin called on his Tesla stock, but I do think him borrowing against it is a significant risk for Tesla. In my personal opinion, Tesla's biggest asset is Musk's fan base - I think it would really significantly impact Tesla if Musk got margin called and couldn't (or doesn't) cover and lost his status as the largest shareholder of Tesla. I think he would lose more interest in the company and you risk his fans no longer seeing Tesla as "his" company.

I don't think I'm alone in this opinion either, Musk has borrowed enough against his shares to warrant getting margin called mention as a risk in the 10-K. (Not to say its wrong for him to borrow against them, I'm sure I would do the same - I just think doing it to fund a private company he is heavily involved in (and distracts him from his public company) links them financially in a way I would personally not be comfortable with if I were an investor in Tesla).