Bent Shift Fork: The shift fork is a component inside the transmission that moves the gears into position. If the shift fork is bent, it can cause the gearshift to slip out of gear.
Synchronizer Problems: Synchronizers are components inside the transmission that help to align the gears before they engage. If the synchronizers are damaged or worn, it can cause the gearshift to slip out of gear.
Low Transmission Fluid: Low transmission fluid can cause the gears to grind and can also cause the gearshift to slip out of gear.
Transmission Control Module (TCM) Problems: The TCM is an electronic control unit that helps to manage the shifting process. If the TCM is damaged or malfunctioning, it can cause the gearshift to slip out of gear.