In this work, we focus on the experimentally well-documented MIL-53(Al) material known for its reversible switching between a large pore (lp) and a narrow pore (np) form upon gas or solvent adsorption. Interestingly, it has recently reported the occurrence of the lp to np transition without a guest molecule, establishing the intrinsic bistable behavior of the MIL53(Al) host. While previous simulation studies have examined the role of guest molecules in the lp to np transition, the driving force for the formation of the guest-free np structure has yet to be elucidated. More generally, the question of the origin of the intrinsic bistability of this topical MOF remains open. Here, we show that dispersive interactions cause the np structure to stabilize at low temperature and entropy drives the structural transition to the lp phase.