Why is the wing bending so much in this video?
Because that wing is a cantilevered beam which is now supporting over half the weight of the jet on it.  This causes the wing to flex, much like a leaf spring, when loaded.  While fascinating to watch this is benign and designed to do this.  In addition, this wing flexure helps to absorb bumps, air turbulence, dampen flutter, maneuvering loadings, etc. much like the coil springs in your car's suspension do.  Don't be alarmed to see this.  Those wing structures have been tested to far worse loads and conditions that you will ever see on a revenue carrying flight before failure occurs.  A 747's wingtips can flex by as much as 12ft up/down during flight.  Newer composite wings are capable of much greater flexure.

This was video shot of Boeing's destructive testing on the 777 wing.  Note the wingtip flexes by as much as 20ft upward at a loading over 150% that of its design load limits before structural failure occurred.