“Chofetz Chaim – A daily companion – Day 74
” One should not praise someone in public,” says the chofetz chaim. This is because the law of averages dictates that there will be at least one person who either is jealous of the person or has something against him – in which case the praise is sure to set off a negative reaction.”
By negative reaction, it is referring to someone speaking badly about the person. This halacha is brought from Klal 9, seif 2. If you look it up, you will see that the Chofetz Chaim writes is that it’s a problem if someone might speak about him negatively. He also gives an example of a case in which someone thinks badly about him but doesn’t say anything, and he says that’s not a problem.