All of the above.
Poor management.
Early on, Yellow management failed to exercise due diligence. They should have studied the failed Associated Transport/Eastern Freightways merger. They should have looked at the problems with the ABF/Carolina merger that almost brought them down. Recognizing that neither Yellow Freight nor Roadway would survive alone made sense. Yellow's decision to grossly overpay with borrowed money was a huge mistake.
Deregulation.
The regulated industry closed the market to competition. Government controlled freight rates allowed poorly run companies to pass on the costs of inefficiency to the consumer. Companies had no incentive to control costs and allowed labor costs to soar to avoid work stoppages. Deregulation created a free market system of competition. Many companies were unable to adapt and shut down.
Labor.
Labor refused to accept the changes brought about by deregulation and continued to demand unsustainable compensation packages of wages and benefits. The consumer based American system of economics worked. Competition forced the industry to provide exceptional service at reasonable prices. Those companies, hundreds of them, that could not compete, closed.