Last Hitting Causes Boo's?

I always wondered why when playing in the bot lane sometimes you hear like a crowd "booing" when attempting to last hit?

I am not sure if it is because of missed last hits or what, which is why I am asking.

Anyone know?

Here is an example, I guess maybe it is only with Thresh? https://www.youtube.com/watch?v=HdOTs8U2a0Y at 4:35ish. You can hear a "oooooooo" kind of sound. That's what I was talking about lol.