A strike is a war of attrition. Employees collectively refuse to work in order to hinder or halt the productivity of the company. Whoever runs out of resources first capitulates; in a successful strike, a company will negotiate terms that are more favorable for the employees because it is cheaper than the lost revenue of holding out.
What if the same war of attrition is initiated by the opposing side? Companies refuse to hire, simultaneously causing financial insecurity to those unemployed (lest anyone mention the low unemployment rate, I consider the 44% of workers who do not make a living wage to be a part of this group) while also resulting in a loss of productivity for the company. As with a normal strike, the intent is to outlast the other side until they capitulate to the demands of the other side. In this case, we see the same job openings hiring for significantly less than they were 2-3 years ago. (To be fair, a significant counterargument about this being a war of attrition is that companies have also boasted record profits the past several years, so it's kind of like playing chicken except one side doesn't get hurt if you crash into each other.)
While explicit collusion is possible, this practice doesn't even require deliberate communication on the part of companies, least of all once the snowball effect is in motion. All it requires is for the company to see the writing on the wall and decide they want to get in on the action. The more participants, the greater the yield.