1. Insufficient tightening 

Bolts that are under-tightened or falsely tightened inherently have insufficient pre-tightening force. If they become loose again, the joint will not have enough clamping force to hold the various parts together. This can cause lateral sliding between the two parts and the bolt will be subject to unnecessary shear stress, which can ultimately cause the bolt to break.

2. Vibration

Tests of bolted connections under vibration have shown that many small "lateral" movements cause the two parts of the connection to move relative to each other, as well as to the bolt head or nut and the part being connected.

These repeated movements counteract the friction between the bolt and the parts being connected. Eventually, the vibration will cause the bolt to "rotate loose" on its threads and the joint will lose its clamping force.

3. Embed

Engineers who design and develop bolt tensions allow for a break-in period, a certain loss of preload, during which the bolts can loosen in tightness.

This loosening is caused by the embedment between the bolt head and/or nut, threads, and mating surfaces of the connected parts, and can occur in soft materials (such as composites) as well as hard polished metals.

If the joint is improperly designed, or if the bolts are not initially tensioned to the specified tension, embedding of the joint may result in a loss of clamping force that does not achieve the minimum required clamping force.

There are microscopic unevenness between the joint surfaces. Under the action of the bolt pre-tightening force after tightening, the bumps will be crushed and permanently plastic deformed. As a result, the clamping length of the bolt will be reduced, ultimately leading to a decrease in the pre-tightening force of the bolt.

4. Washer Creep and Thermal Expansion

Many bolted joints include a thin, soft gasket between the bolt head and the joint surface to seal the joint and prevent gas or liquid leakage. The washer itself also acts as a spring, rebounding under the pressure of the bolt and mating surface.

Over time, especially when near high temperatures or corrosive chemicals, a gasket can "creep," meaning it loses elasticity, resulting in a loss of clamping force.

If the bolts and joints are made of different materials, excessive temperature differences due to rapid changes in the environment or industrial cycling processes will cause the bolt material to expand or contract rapidly, possibly causing the bolts to loosen.

5. Shock

Impact - The larger impact load exceeds the friction force when the bolt is pre-tightened, causing sliding.

Dynamic or alternating loads from machinery, generators, wind turbines, etc. can cause mechanical shock—an impact force exerted on a bolt or joint—causing the bolts to slide relative to each other.

Just like vibration, this slippage can eventually cause bolts to loosen, and even shocks are often not considered when designing joints for such large loads.