Was it sabotage, or accident?

Not necessarily. When a plane is flying, there is much more stress on its structure than when it is standing on the ground. This is especially true at take off and landing. Bolts and other parts can fail in flight but look perfectly fine on the ground. Engines do fall off from time to time, as a quick Google search would have told you. Sometimes this is because the engine has seized due to sucking in an object.

In fact the engine mountings of most if not all commercial jet aircraft are designed to shear so that a damaged engine will fall off rather than endanger the plane.

There are several reasons for this.

1. Catastrophic Internal Failure... If this happens, usually the engine will develop a major vibration that could literally shake the plane 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. This structure could be the wing front spar or (in the case of the 727/DC9 and their families) the same structure that supports the entire tail assemblies.

The shear feature is usually built into the actual mount bolts, or into a linking feature known as "Fuse Pins". Many of these pins and bolts are today covered by Airworthiness Directives, which command inspections, torque checks and replacements at scheduled intervals.

Since the planes was designed with these features in mind, the engineers design the aircraft to fly with an engine missing.

That should be:

Since planes was designed with these features in mind, they can be flown with an engine missing.

Very informative. I did not know that.
Thanks