Keep in mind that the layoffs followed a period of massive hiring, and in many cases the companies still had more employees post-layoff than they had pre-hiring-spree.
Unions would likely prevent this behavior (hire 10 then layoff 2) because companies with unions are much slower to hire. Instead of hiring 10 when things were going well and they were optimistic, they might hire only 1 or 2. So you'd have less layoffs, but also less people employed.
Also, unions can prevent layoffs in the short-term, but keeping the deadweight employees harms the company and the increased employment costs can make it so that a company doesn't survive a lean period and instead goes bankrupt, making everyone lose their job.
