That's because his bonus was probably tied to your performance. By making sure all his subordinates receive meets or exceeds expectations, then he looks good. His manager does the same, all the way up the chain.
They played the same game when I worked at Amazon. What's more, it became automated. They introduced non-optional surveys that popped up on your computer daily. At first I assume it was a well intentioned system to gauge general employee sentiment. It was annoying and stupid HR bullshit, so of course I immediately went in and disabled it. After a year or so, my manager finally notices and orders me to enable it again. I soon guessed why. Within a few months, we start having quarterly group meetings going over graphs of the answers. And of course, the surveys aren't anonymous, so he would call out the people who gave bad answers and start grilling them about their issue in front of everyone, if they didn't immediately recant, then they would "schedule a meeting". I assume his performance bonus had become tied to the results and everyone needed to tow the line. It was amusing to me how many of the younger employees didn't understand the game they were playing and would continue to answer honestly. I just glanced at the options, picked whatever made my manager look good and went on with my day.
You'd think those idiots in charge at the upper management levels would have heard of Goodhart's Law: "When a measure becomes a target, it ceases to be a good measure." But apparently not.
