Here is part two when they return and it’s still going strong.
Several of the commenters on YouTube and on Live Leak have hit on what the answer is to this odd behavior. Notice a few things: the shape of the leaf and the slight breeze that is moving the vegetation behind this plant. It sticks straight up and has an airfoil structure. Therefore it is catching the slightest of current which perhaps is it’s resonating frequency making it… resonate. The same thing happens, unfortunately, with bridges. Other commenters noted they have observed this in plants in their areas as well.
Here is one example of someone explaining the physics:
[...]It has characteristics that allow air pressure to be lesser on the windward side and higher on the leeward side, thus creating movement in a direction only held or forced by the fulcrum, which in this case is the plant stem itself keeping it adhered to the ground and thus causing mandatory motion with no where else to go.
-via LiveLeak.com – Mysterious swaying plant.
Also, I noted in the comments that people GOT MAD when the explanation was given. They wanted it to be a creature signaling you from underground, a haunted forest, a nest of spiders, etc. Why can’t this really amazing and fascinating thing be appreciated for what it is – nature just doing something weird and beautiful.
And, just to note, because we study such things, there aren’t any Tacoma Narrows bridge disasters anymore. Yay science.