How does this happen? It happens because you accidently hit the "command", "shift" and "L" keys which is the keyboard shortcut for reading [L]ist. Hit those three keys again and the drawer will close.
There are other ways to do this, namely go to "Safari" drop down in the menu bar, select "Preferences..."
In the preferences panel select the first tab, "Bookmarks". The top checkbox on the Bookmarks panel is the one to include the "Reading List" symbol (eyeglasses) on the bookmarks bar (far left end by default). Check that box and then click on the eyeglasses in the bookmarks bar to make it disappear.
Alternate way 2 is to click on the "View" tab in the Safari menu tab and select "Hide Reading List".