Let's say it's for something petty-ish. Like they grabbed the last mango at the supermarket that you had your eye on and were walking over to get it. Idk you're the witch so you can choose your level of vindictiveness. Explain what they did if it's fitting in with the curse.
make them only able to speak backwards for the rest of their days, so that, if they want to speak forwards, they need to make a conscious effort to talk backwards.