Heat - increases the resistance of the circuit thus increases the current which cause the material to expand, dry out, crack, and wear down much quicker and sooner or later the device will breakdown.
Molsture - makes circuit to draw more current and eventually breakdown. Moisture like water and liquids cause expansion, warping and abnormal current flow or short circuit.
Dirt and other contaminants - such as fumes, vapors, grease, oils etc that cause electrical device to clog or gum up and operate abnormally until breakdown
Vibration - and physical abuse can cause also these types of breakdown
Poor Installation - which often work of unqualified personnel who is careless or in a hurry can cause also breakdown soon.
