Some of the reasons might be that your employer:
Did not pay you
Demoted you suddenly and for no reason
Forced you to accept unreasonable changes to your terms of employment
Allowed other employees to bully or harass you
Took away benefits such as a company car, for no reason
Forced you to work in unsafe conditions
Did not give you support that you needed to do your job
These reasons can be a one-off event that triggers your resignation or a pattern of events over a longer period.