The mount bolts and/or mounts themselves have a shear feature. There are a few reasons for this, mostly..
1. Catastrophic Internal Failure... If this happens, usually the engine will develop a major vibration that could literally shake the airplane to pieces. Better to have an engine drop off than to have the entire airframe come apart.
2. Externally Induced Damage... If something hits the engine so hard that something has to give, it is better to have the engine break off than tear away the structure that the engine is mounted to.
The shear feature is usually built into the actual mount bolts, or into a linking feature known as "Fuse Pins". These pins and bolts are covered by Airworthiness Directives, which require inspections, torque checks, and replacement at scheduled intervals.
For some reason, I seem to remember talk about 'explosive bolts" that could shear the engine off...but dont think that is the case, or is a feature that can be commanded by the pilot or system....