Amazon is all about the money. They fully admit to firing people based on a computer algorithm. They also are a huge company, with over 600,000 employees. With that many employees, you’ll end up with a lot of pregnant women–each woman has a manager. All it takes for a lawsuit is for an employee to feel that they were treated unfairly and for a lawyer to be willing to take the case.
While some employment lawsuits result in huge payouts, most don’t. But Amazon also has deep pockets, and where there’s one employee complaint, you’re likely to find others (just given the sheer volume of employees), so they can be a good target.