This is based on the discussion here; I don't understand much of the underlying technical details unfortunately.

It seems like setting -Crelocation-model=rwpi on an ARM target compiles code in a way that it expects a particular register to be reserved for data addressing. However, the standard library is not built with that in mind and can use the register for other purposes. That's clearly unsound, we can now get arbitrary misbehavior because the same register is used in conflicting ways.