A corporate sabotage version is unbelievable. What if it comes out? Why do it from their own building?.. Won't believe it until an official admits in court they did it, and would doubt even then.
However, what about an employee going postal and acting on their own, out of envy, hatred, or fear of being laid off? It looks like it's possible to buy a suitable rifle in USA (I'm not a resident), so if only they could bring it to the building... And a ULA employee *should* be able to know when to shoot, and what happens if LOX equipment is hit.
That said, the chance is still beyond slim. On the other hand, making unsound sabotage accusations in this circumstances would be a PR suicide. SpaceX better have rock solid evidence if they do.