The author thinks it's because those people are more stressed. I think it's because those people are sending a message to their boss that they are a pushover.

That totals $52.4 billion workers just gave away. Essentially, rather than take the time they were due off, millions of American workers said to their bosses, “Hey, no problem, we’ll work for free.”
https://www.linkedin.com/pulse/artic...d-for-business