Why does this vortex bubble ring reflect off the water-air boundary?
I think this diagram represents what ought to be happening (I invite others to comment / critique / improve):
It depicts a section through the vortex ring, moving from left to right. The upper part is rotating counterclockwise, while the lower part is rotating clockwise. As the ring gets closer to the surface, the water being pulled by the upper vortex will accelerate, and due to Bernoulli the increased flow velocity lowers the pressure. This should result in dissipation of the vortex ring - not reflection.
In the limit where the surface is considered to be rigid, you can apply a boundary condition that can be resolved by stipulating the presence of an "image vortex" that ensures there is no flow normal to the surface. But such a vortex would again tend to dissipate (rather than reflect) the original ring.
A lot of information is given in "Vortex Ring Interaction with a Free Surface" by Song et al, in the proceedings of the 18th symposium on naval hydrodynamics (1991(. In their experiments, the vortex ring dissipates when it reaches the surface.
All this says - the observation in the video clip is anomalous, this is not what you would expect for reflection off a still free surface. But then this is a pool, and there are waves (ripples) on the surface. I suspect that these "hit the vortex ring just right" so it was reflected rather than dissipated. It would be interesting to do this experiment many times, with a still surface. I suspect the result would be different.