Most fractures and/or dislocations are the result of significant trauma to bones and joints.
The bony structure may be disrupted by a variety of forces including a direct blow, axial loading, angular (bending) forces, torque (twisting) stress, or a combination of these.
The urgency of treating dislocated joints (reduction and immobilization) with emergency splints is due to -

Potential for neurologic or circulatory compromise - the neuro- vascular bundle passing close to the affected joint is typically 'kinked' around the deformed joint.

The longer a joint has been dislocated, the more difficult it may be to reduce and the more likely it is to be unstable after reduction.