What are the common reasons a company does a mass layoff?

due to limited resources

They don't want to pay the workers anymore

Costs, money. Usually the almost instant, go to short-term "solution".

I believe when a company does a mass layoff the reasons a commonly because of the high salary and low productivity.

Buyouts, business re-locations, or failure of the business to adapt to the market.

the CEO spent the money on themselves

They either run out of money, decide to close a business unit or get bought and the new owner eliminates redundant staff.

Tariffs, increase in costs of materials, insurance, imports or decrease in exports, customer spending, depression, recession, any number of things.

They're not doing as well as they led the employees to believe.

Trying to increase their revenue per head.

Some common reasons a company may do a mass layoff include the company needing to cut costs in some way, overstaffing, or if a company is doing a complete relocation.

Lack of real strategic planning, quarterly metrics for investors rather than stability for employees

They have incompetent retards reaching up the executive levels with no accumen for vision and strategic business

A bad fiscal year, mergers, buyouts, leaving a product market, modernizing with automation.

Headcount reductions are cost cutting measures to stay comparable to competitors or could be the result of acquisition.

Duplicates in work flow and too many managers and not enough people doing the actual work.

Reduce financial burden

Budget cut, greed on upper management

Poor planning leads to massive, quick hiring frenzy to ship product, and then we let them go when the product has been launched. It's hard to develop a good hiring procedure, and it's easier to periodically lay people off instead of admitting and fixing the real problem: not knowing howto grow a business in a sustainable way. Frequent reorganization of upper management that exists one or more layers about the hiring managers. A way to seemingly improve the bottom-line before the fiscal year is over (for publicly traded companies).

Poor management... penny wise pound foolish.

Change of corporate culture.

Poor company performance; general restructuring; new leadership

Major economic issues and hopefully as a result of finding no other alternative to ensure viability of the company. Employees are the highest expense as much as they are the most vital asset the company has. Leaders have an ethical moral imperative to only resort to reduction in force after all other means of managing financial woes are exhausted!

They are primarily used as a myopic way to quickly increase booked profits. Labour and related expenses are usually the highest cost center in an organization. If revenues fall, you can quickly layoff a lot of people and still have on paper profit. In reality, though it will snap back later in terms of employee dissatisfaction, increased hiring costs, quality degradation, and community sentiment. But at the time no one making the decision cares, as the primary goal is to post profits to the street to prop up share prices.

Shareholders loss of dividend, new regulations in area where company operates, competition caught up and company was unable to maintain market share

This is a complex question with a complex answer that would need some perspective and detail. Mostly restructuring to reduce debt or eliminate under performing departments. They could be moving away from a specific technology and that would involve a RIF.