It's easy to file a lawsuit. You have to claim something mildly plausible that the court recognizes as possibly legally valid. At that point, the company has to burn lawyer time in order to fight the case. Even if they win, it cost them. (It cost the ex-employee, too, but the company doesn't care about that. They only care that it cost the company.)

Maybe I'm cynical from watching SCO v IBM (still not over, after 16 years, even though SCO never really had a case), but my perception is that you can file a lawsuit without a case. You just have to, in your initial filing, give the appearance that there might be a case.