1- Incorrectly stored chemicals that could by mistake mix together and cause an exothermic chemical reaction.
2- Air-conditioning units that may be left on for long periods of time in the summer months becoming overheated. 
3- Substandard electrical installation such as overload, heating from bunched cables, or damaged cables.
4- Charging units such as those used for forklift trucks. 
6- Employees smoking inside discarding cigarettes carelessly. 
7- Hot light bulbs too close to packing boxes and other stored goods. 
8- Hot processes,e.g. welding by contractors.
9- Arson.

Also
1- Poor fire separation through the building may lead to very rapid fire spread. 
2- Naked flame, e.g. gas or liquid-fueled open flamed equipment. 
3- Heated extract fans for dust and fume removal systems. 
4- Failure of temperature control thermostat during hot processes.  
5- Heat sources such as gas, electric, microwave, thermal fluids. 
6- Hot steam pipes. 
7- Frictional generated from mechanical equipment. 
8- Static charge generated from mechanical equipment.