Spooky Playground - how "ghost' swings can move on their own

So a friend of mine managed to record a video of a swing that was moving all on its own. There is no trickery here, this actually happened. The video is below.

Now what can cause such a thing? There are a few possibilities:

Vortex Shredding: Has caused buildings to collapse, its periodic low pressure areas at a particular point.

Aeroelastic flutter: Caused the collapse of a bridge, a consequence of the object having an aerodynamic shape is that it starts oscillating.

Crosswinds: two or more gusts intersecting at the exact location of the swing.

Mechanical resonance: Slight oscillations matches natural frequency of the system, causing it to sway.

I'm not an expert, and do not know much about this, just putting together a list of possible explanations.