A lot of the time, it's just that *they* are not ready for their kid to go to middle school. Even the parents of smart kids. Of course, if they really aren't academically ready, then that's a different story. They know, deep down it's not your fault, they just can't admit it.
I had a wonderful article written by a columnist in the Boston Globe many years ago, that described how parents feel at this stage. They worry about everything. I used to copy it and send it home around this time of year.