Previously, when you show to a tribunal or whatever institution that they fired you for not working out of hours despite insisting it was some vague "job performance" reason, the tribunal says well, that's technically legal anyway, it's just rude, so too bad.

Now, it's illegal. So "some other reason" has to be watertight. If in the process of concocting a "some other reason" you trip another law, you don't get a Do Over because you were trying to break a different law, instead you have more trouble.