Quiet quitting is a recent name for something I am sure has been happening for all of time. It probably has something to do with the person's life circumstances, or something about the macro environment being depressing, or whatever. But let me propose a simple economic justification.
Consider the total impact of employment on a person. That includes their salary and benefits, commute, how much they like the work itself, colleagues, etc. Overall, this forms the "deal" inside the employee's head. On average, when you join a company you get a fair deal (relative to your market opportunity, which itself might feel unfair) because you pick your best opportunity.
It's nice when the deal gets better: you get to keep doing the same thing but your life is better. However, when the deal gets worse (e.g. with high inflation and no pay raise), you're a bit stuck: job switching costs are generally high (especially in situations where alternatives require relocation or lifestyle change). This means there are significant periods -- definitely months, and perhaps years -- after the deal got worse but the employee hasn't quit yet, where the employee is getting a bad deal.
A bad deal and no way out is fertile soil for resentment, which (I observe) is what often happens in the pre-quitting stages of employees.
But resentment doesn't compensate for the bad deal. In fact, it makes it worse! Not only do you have a bad deal in objective terms, you also feel worse.
To preserve the value of the deal you had before, you need to compensate. Here are a few ways people compensate:
- Working less: literally showing up for fewer hours of the day. Sometimes it is very low-cost, e.g. with remote work or flexible schedules.
- Working less intensely: you might still show up and leave at the same times, but simply decide to take the pace down a notch. Maybe you play Candy Crush during video calls. Or watch a TV show while coding.
- Choosing to accept less stress: this could take many forms. You might volunteer for fewer things, or actively try to avoid getting tasks assigned to you.
- Literal stealing: from taking pens and fruit home, to perhaps downright stealing cash, embezzling, taking bribes.
If you see marks of the above in an employee (especially in a tech company), I don't think salary is the main lever to change it -- it won't last. But perhaps it makes sense to ask in the next 1:1 whether the employee thinks they are getting a fair deal.