Outing is when someone discloses someones gender identity and sexual orientation without the person's consent. This is wrong because when someone comes out to you they trust you with this sensitive information and it take a lot of courage for someone to come out. Outing someone is a breach of trust and privacy and is condemned.
Sources:
https://www.lgbtqandall.com/what-is-outing-and-why-is-it-harmful/