If you monitor daily chart in real time you will see that as soon as there is a large block of sell, immediately there is a buy order that kicks in to compensate for the drop and brings the price back into a pre-determined range. Today's morning range has been established between $17.08 and $17.02.
It is all automatic, and high speed computer algorithms execute these trades. If you have an engineering background, you will notice by looking at this morning's opening one hour that it followed a second-order feedback control system response, where initial transition was followed by an oscillatory trend and finally settling into a steady state range. All very sophisticated computer controlled trading by the big guys.