From this you can see that smaller bubbles have a higher difference in pressure at a fixed liquid surface tension. Perhaps malted wheat causes surface tension to lower. Maybe someone could measure the differece in surface tensions for equal-gravity worts, one made with wheat and one with normal barley. Also, perhaps the high-serve pressure of pilsners, etc. helps makes the fine head. The driving force behind bubble coalescence is really thermodynamics.
Bubbles have a certain surface free energy that always is minimized.
This is how one actually can derive the above given equation from thermodynamic principles. Anyway, when two bubbles collide and stick, the total surface free energy is not at a minimum. From the above always heading toward minimum total free surface energy idea, the surface area shrinks by the bubbles' walls merging to a new, larger single bubble, and the total surface free energy is minimized. Then the temperature is raised slightly and the pressure let up so that the beer is supersaturated with N2 in solution. This is probably similar to a liquid having more that a soluble amount of solid dissolved in it where the temp is raised - more is dissolved, and the temperature is lowered again without the excess dissolved solid material crashing out of solution.