Pallas

On topic of 10 Hygiea's origin, let's ponder other rocks. Vesta's probably always been there; Ceres like Hygiea is an interloper. So: 2 Pallas.

2 Pallas is about Vesta's size but, since we've sent an orbiter around Vesta, we now can constrain that one at least. 2 Pallas is actually slightly smaller and 79% the mass. It shares its orbit with Ceres excepting its monstrous inclination. Pallas like Ceres are 18:7 with Jupiter so chaotic - especially if we add other major planets.

Problem: Pallas (unlike Ceres) looks like it belongs here. It is carbonaceous, "B" subtype (for "blue"). At 2.77 AU we do seem to be in the sooty layer, outside Vesta's stony origins.

I suggest that Ceres, muscling into Pallas's near-turf, forced Pallas into its orbit such that they'll never crash. Pallas may have formed (slightly) inside where it is now, lending momentum to Ceres. Or maybe Pallas formed outside but crashed into something else, dragging down its semimajor. Anyway now they are protecting each other from the secular forces of the other planets.