Ideas that have been suggested include 1) perturbations from a closely passing star 2) perturbations from a more distant (undetected) planet, that might or might not still be present 3) perturbations in the early solar system resulting from planet migration and heavy mass loss from the Kuiper belt.
The first known example was 2000 CR105 (perihelion distance = 44 AU). The best-known example is 90377 Sedna (perihelion distance = 76 AU).