What is the main cause of grievances in the workplace?

The most typical cause of grievances in the workplace is disciplinary action taken by management against workers. In a workplace, the managers must sometimes take actions against their workers.  They may do so because the worker has been doing bad work or because the worker has been breaking rules (such as coming to work late).  It is often the case that workers who are disciplined in this way feel that they are being unfairly targeted.  They often feel that the manager has something personal (as opposed to work-related) against them that has led to the discipline.  This feeling often leads to the filing of grievances.

Reasons for employees to think in terms of "grievances" or "unfair treatment", whether real or perceived as such are:

1. Bad (conflict) management.
2. Wide-spread nepotism & friends politics.
3. Authoritarian management styles and business cultures.
4. Bad communication.
5. Personal employee reasons: not to indemnify employees entirely, the reason for such perception may lie within them, themselves. It could be an employee's insecurity, sense of not fitting in or belonging to the team, envy of (better or more liked) others & their tendency to relate such decisions to their inner (irrational) feeling that people generally are unfair to them.

Everyone has their eye on the prize but everyone can't get it.

It's a misture of personal and professional jealousy usually.

Lack of Leadership in the workplace.  If there is no leader to draw the path and create leaders to achieve the goals then the workplace turns into hell.  All other things like jealousy, misuse of authority, micromanagement, favoritism, ......all these things are a bi-product of the lack of leadership.  Leaders engage everyone and inspire people, they bring the best of everyone and create leaders out of each and everyone.  And leaders do not have time for all these small things, they dream and plan and achieve instead.

Incompetent and directionless leadership,poor hierarchical pattern and poor grievance handling procedures

Absence of justice

Favoritism and unequal opportunities 

1. Bad management 
2. Bad working environment
3. Under-paying salaries

Grievances are usually caused by a "disconnect" between management and staff, or between the company and an individual.  There are a lot of unbending people out there, unfortunately, who rigidly enforce rules, whether it's humane or not.

Generally the problem festers for ages, damages morale, and causes dissension until someone finally writes a complaint about one specific issue and grieves it.  If you talk with that person and their co-workers there is usually a lot more going on, and none of it is good. Look at the grievance as a cry for help.

Ineqality and unfair award of comensation & rewards including promotions & increments to employees or workmen in same position, with similar job responsibilities.

Politics at work...