What makes a union worker qualified to bump another union worker out of their position?

In this week’s chapter readings we are introduced to a myriad of labor laws in the United States that are intended to provide reasonable stability in the labor relations area. The current focus in many large organizations is to go global which can interpret to downsizing, re-engineering and in general, a reduction in unionized employees.
This week’s discussion focuses on the need for additional laws to protect employees from being displaced or treated unfairly by employers.   With a collective bargaining agreement in place, union workers will enjoy some job security.  For example, the collective bargaining agreement may  state that downsizing or layoffs will be done by seniority in such areas as job classifications or seniority from the date of hire.  The contract may call for bumping rights for the union worker facing a layoff or downsizing.  Bumping means that the worker being laid off can bump a worker with less seniority as long as the worker qualifies for the position.
The larger question for union workers is whether “seniority” should always be the rule.  Along with this idea of seniority, is it seniority from the date of hire or job classification seniority?  Which seniority should prevail?  What about bumping?  What makes a union worker qualified to bump another union worker out of their position?  Can this person who is bumped find someone to bump if the union worker meets the qualifications?  How far does bumping go throughout the company? In spite of a contract, do these areas impact the company?
I hope that you feel able to respond.  If I have explained the seniority concept and the bumping concept, please respond in the way that analyzes these scenarios.  You can also consider management’s concerns as well with the idea of seniority and bumping.