If the system is NEW then the following points apply:-
* Bad ventilation around CPU case.
* Insufficient cooling equipment (extra fans or coling kit will be req if CPU has a GFX card or etc)
* Blockage of airflow regions like behind SMPS etc.
If the system is OLD then the following points apply:-
* Dust accumulation on fan or heatsing
* Might need new thermal paste on CPU heatsink
* Fan stuck or slow due to old age (worn out fan)