Excessive loading can cause overheating. Transformers are rated in kVA. This kVA rating is dependent on temperature. Some transformers have two kVA ratings. One rating is for below a specific ambient temperature, say 45 degrees while another rating is above the ambient temperature.
Excess current in the neutral of the transformer
Excess current in the neutral of the transformer is usually caused by high zero sequence harmonic components. This can be resolved by proper grounding usually through a zig zag transformer (See article).
Problems in the Cooling system
Malfunctioning of the cooling equipment such as blockages in the cooling oil circuit. Failure of the cooling fans to operate in the case of Forced Air cooling systems.
High Harmonic content in the power supply
High harmonic loads can also cause heating in the transformer. These loads need to be identified and suitable remedial measures such as harmonic filters can be implemented.
Overvoltages which exist for a long period of time can overexcite the transformer and cause overheating.