A derailment occurs when a vehicle such as a train runs off its rails but does not necessarily leave the track entirely. Derailments result in temporary disruption of railway operations and pose safety hazards. Derailments can be caused by collisions with other objects, mechanical failures of tracks like broken rails, or mechanical failures of wheels. Common causes of derailments include primary failures of track components or vehicle parts, faults in track or wheel geometry that cause static failures, dynamic effects from track-vehicle interaction like excessive speed, improper operation of switches, or secondary events following collisions.

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Derailment
A derailment is said to take place when a vehicle (for example a train) runs off
its rails. This does not necessarily mean that it leaves its track.
Although many derailments are minor, all result in temporary disruption of the
proper operation of the railway system, and they are potentially seriously
hazardous to human health and safety. Usually, the derailment of a train can
be caused by a collision with another object, the mechanical failure of tracks,
such as broken rails, or the mechanical failure of the wheels.
Causes
Derailments result from one or more of a number of distinct causes; these may
be classified as:
– the primary mechanical failure of a track component (for example broken
rails, gauge spread due to sleeper (tie) failure)
– the primary mechanical failure of a component of the running gear of a
vehicle (for example axlebox failure, wheel breakage)

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– a fault in the geometry of the track components or the running gear that
results in a quasi-static failure in running (for example rail climbing due to
excessive wear of wheels or rails, earthworks slip)
– a dynamic effect of the track-vehicle interaction (for example extreme
hunting, vertical bounce, track shift under a train, excessive speed)
– improper operation of points, or improper observance of signals protecting
them (signal errors)
– as a secondary event following collision with other trains, road vehicles, or
other obstructions (level crossing collisions, obstructions on the line)
– train handling (snatches due to sudden traction or braking forces).

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Sources: http://railtec.illinois.edu/CEE/pdf/Journal%20Papers/2012/Liu%20et
%20al%202012.pdf, wikipedia.org