Generally I think these scenarios happen because the founders get backed into a corner and forced to exit with a sub-optimal deal:
Founders don't _want_ to screw over employees, but at the end of the day, they have a duty to their investors too and if the company is failing, the best thing can do is recoup something out of it.
Outcomes for employees are based on decisions well before the exit.
I suspect some founders just never really get around to the up-front tasks that are necessary to take care of employees. It takes significant effort to put together an employee option plan. You also have to go to the mat for employee equity in funding negotiations, for example to top up early employees. It's easy to slight these in favor of tasks that contribute more directly to company success.
Founders don't _want_ to screw over employees, but at the end of the day, they have a duty to their investors too and if the company is failing, the best thing can do is recoup something out of it.