I think this paragraph from Wiki explains it, though I'm no expert on this. Like platinum and palladium, rhodium is a very rare precious metal usually found in elemental form in deposits of gold and platinum. Its rarity combined with how it's mined can cause it to get squeezed.
A lot of Americans are using their stimulus money to buy non-cash assets. Metals, collectibles, guns/ammo, etc. With another big stimulus coming. And gold around $2K / oz, this seems like the reason. PS, Wikipedia claims that South Africa is the largest exporter at 80% of world's supply. That could be another reason, being South Africa.
"Owners of rhodium a metal with a highly volatile market price are periodically put in an extremely advantageous market position: extracting more rhodium-containing ore from the ground will necessarily also extract other much more abundant precious metals notably platinum and palladium which would oversupply the market with those other metals, lowering their prices. Since it is economically infeasible to simply extract these other metals just to obtain rhodium, the market is often left hopelessly squeezed for rhodium supply, causing prices to spike. Recovery from this supply-deficit position may be quite problematic in the future for many reasons, notably because it is not known how much rhodium (and other precious metals) actually was placed in catalytic convertors during the many years when manufacturers' emissions-cheating software was in use. Much of the world supply of rhodium is obtained from recycled catalytic convertors obtained from scrapped vehicles. As of early November 2020, the spot price of rhodium was US $14,700 per troy ounce."
https://en.wikipedia.org/wiki/Rhodium