Because someone, who should've chosen to be more professional, broke the trust of a confidential conversation. This whisper campaign only serves to compound the broken trust and lack of professionalism.
Folks, we're talking about someone's performance and livelihood here. Who wants their last performance review debated on-line?
If there's stuff in the public square that's readily pointed to, please do. If there are concerns best left to be managed through a managerial chain of command, let that closed-door chain work. If you aren't sure which is which, think about what you would and would not want debated here from your last work review.