I've been on the employer side of this... you fire someone who's performing badly, and then they come back 4 months later and sue the company for [insert made up thing here].

In our case, an ex-employee is suing us for not accommodating an anxiety and migraine disability, which they never disclosed and never requested accommodations for. So now we face a discrimination lawsuit (from a non-minority) based completely on falsehoods and things that never happened.

The reason people do this is because it works! Employers will almost always settle before it goes in front of a judge in order to avoid the hassle and cost of defending the claim.