I paid for a few years, but basically went back 20 years and copied every article that was remotely interesting and organized them by subject matter....
So based on the information provided by Cory, the OP, it is the drawer front which is flexing, understandable I guess since it is so wide. As others...
I guess I am not understanding how the sides of the drawer would flex if the corner joints (dovetail, rabbet, finger joint, etc) were properly made. ...