Why things wobble when submitted to wind? If I place a little piece of paper in front of a fan, it is going to wobble/oscillate.
Is it because the wind flow isn't constant ? Or because the air movement makes the piece of paper move away, so it doesn't offer the same resistance, so it comes back, and then the air can push it with more strength again ?
 A: The wind flow does not need to be variable. Things can still wobble because of turbulence. If you could visualize the airflow around the object, you could see that it is not a smooth flow, but a flow with vortices. These vortices form near the object but then detach from it. The force acting on the object is different while a vortex forms vs. when it detaches.
Then there is aeroelastic flutter (see https://en.wikipedia.org/wiki/Aeroelasticity#Flutter. I naively thought that it is no different from vortex shedding, but I stand corrected. I thank Glen the Udderboat for pointing my nose in the right direction.) This is the response of an elastic object to a fluid flow, altering its shape and thus in turn altering the force acting on it. This can result in harmonic motion, oscillation, and it can even be destructive.
Remember the famous Tacoma Narrows Bridge, also known as Galloping Gertie? This was the bridge that was destroyed in 1940, shortly after it was built, by wind. (If you haven't seen the video, look it up on YouTube). The wind flow was steady, but the bridge started wobbling nonetheless, just like a strip of paper in front of a fan. This was originally thought to have been the result of vortex shedding, in particular vortex shedding frequency, the rate at which vortices detach from an object. This is approximately linearly dependent on wind velocity. However, if the object in question has a natural oscillation, when the vortex shedding frequency approaches that natural oscillation frequency, a lock-in condition may set in so effectively, vortex shedding and the object itself remain in resonance even if the wind velocity changes slightly.
Here is a nice page with additional details, including plenty of relevant equations and some nice pictures of Galloping Gertie: http://ntl.bts.gov/DOCS/ch2.html
A: The air stream from the fan is created by separate blades that break up the flow and create cells of low pressure and high pressure in the stream.  When the air moves past the paper, it is split into two streams, which push and pull the paper from side to side.  You are right that the paper moves away from high pressure and into low pressure.  This creates a buffeting effect as the paper's stiffness attempts to snap back but is pushed away again.  The restoring force caused by the stiffness of the paper may even result in harmonic oscillation if the cells of varying pressure appear in regular intervals.
But even if the air stream coming from the fan were perfectly laminar, it tends to form streams and eddies as it flows past the sides of the paper.  These create turbulence of the kind that makes flags flutter in the wind.  See Viktor Toth's answer for a description of this.
Navier-Stokes equations (http://www.comsol.com/multiphysics/navier-stokes-equations) are a series of partial derivative equations which are used to model fluid flow and air flow in various situations.  They are notoriously difficult to solve, but they are very useful to model the relations among velocity, pressure, temperature, and density of fluid moving through or past various geometric configurations.  
