Budd–Chiari can be primary (thrombosis of hepatic veins) or secondary (external compression or invasion).

Hepatic outflow tract Thrombosis , (JAK2+), pregnancy, oral contraceptives, , paroxysmal nocturnal hemoglobinuria
External compression , , cysts, abscesses
Membranous obstruction of IVC Especially in Asian patients (can mimic “malformation”)