For some people it takes being fired to leave a job they stopped caring about long ago. It's so easy to fall into a "comfort zone" trap when you've had a job for a couple of years. You stop keeping your ear to the ground for other potential opportunities. You make some acquaintances you have lunch with from time to time and mentally settle into the idea of "riding it out" until you retire accepting whatever raise increases that are offered.
Even if you feel unappreciated or not paid enough in your opinion for some reason this does not cause you to look for a new job.
Instead you start creating your own "perks" to make up for being short changed. Maybe you take a two hour lunch every now and then, take care of personal matters while on company time, come in late and leave early especially on Fridays. You live by the slogan:
"I do just enough not be fired and they pay me just enough not to quit."
One day unbeknownst to you they are monitoring your keystrokes on your keyboard, phone calls, or possibly your comings and goings. Maybe a new boss wants to make an impact by cutting "expenses" AKA (perceived dead weight) in the department or you've withdrawn from everyone and essentially just show up to work and go home. You're not seen as team player.
Either of these could lead to be fired.
The reality is most people (quit) their job long before they are fired.
Several years ago to make a long story short I was in a comfortable rut and even though I had been shortchanged a couple of times regarding (commission structures changes in the middle of year), a part of my territory was given to a woman whom turned out to be the boss's sister-in-law I still didn't take it upon myself to look elsewhere until one day I canned.
I jumped onto Indeed.com and Monster.com and noticed one of my old employers was advertising for a position. I interviewed with a couple of folks there and was hired within one month. Naturally I told them (I resigned) from my old company to look for a better opportunity knowing the HR dept. is only allowed to confirm the position I held and length of time of employment.
Exactly one year from the day I was fired from my old company I was strolling along the beach in the Bahamas while staying at The Atlantis Resort on an (all expense paid vacation) as a reward for exceeding my sales quota where I along with others were given awards at a special dinner. In fact I doubled my income from the previous year!
Had I not been fired I would have missed out!