Because it's a bug. You found a workaround.
Assuming you're using El Capitan, Dave is right. This is a known issue. It's listed on this page and some other places too. You found one workaround. Another workaround found by user mofxtv (in this thread) is to dock another panel under your timeline panel.
Some people use other panels that they use less often.
Thanks! this is a much better work around than what I was doing
You're welcome! The AE team seems to think they've got a handle on the issue according to that thread, so hopefully they'll fix it in one of the next few updates and we won't have to do this workaround for much longer!