Besides mechanical aspects
- bubbles might form a sort of layer at the immersed part while at the air interface they evolve and detache easily renewing the metal/acid (oxidizer) contact
a more fundamental reason can be° surface excess. Normally we treat interfaces as very thin and therefore negligible (in mass or volume) as compared to the bulk.
However at an interface the chemical potential (and all related thermodynamic properties) differs from that of the bulk owing to the fact that a layer can be assumed to have the same number of entities per unit of surface.
While such a layer at depth is surrounded by the very same, at the interface it sees two different environments.
This aspect is quite familiar in physical phenomena, such as surfactivity, the shape of droplets, and even the tip effect in electrostatic. Another example possibly familiar to many chemists is the difference in product of solubility / solubility for solids at very different sizes (digestion of precipitates to attain bigger grains/crystals is based on this, tough intermixed with the merely geometric fact that small particles dissolve faster).
As for in water the interface has higher potential and this results in spherical droplets to minimize it, in your specific case you can see the aqua regia doing the same by being more reactive. It is like the oxidizer trying to escape the air interface (tough a new one is continually formed, of course) by reacting more energically than the layers below, if this pictorial helps you.
If you dig for "potential at acid/air interfaces" or "pH of an acid at the air interface" and so on, you can find some material dealing with example somehow related to your example. For instance I have found study of the dissociation of weak acids (ie pKa at the interface vs bulk), The vast majority is however studies in which the very interface is at focus, like in monolayer LB films, and the surface excess comes naturally at play whatever explicitly considered or not.
(For instance a title such as "On the dissociation constant of carboxylic acids in monolayer thin films" implicitly assumes that some difference exist as compared to the bulk of the solution).
° can be as for I do not know if the evident effect you have spotted is mainly due to the "stirring effect" of evolving bubbles or to the higher activity of the interface, or their mix.