  A series of recent works focused on two-dimensional interface growth models
in the so-called Anisotropic KPZ (AKPZ) universality class, that have a
large-scale behavior similar to that of the Edwards-Wilkinson equation. In
agreement with the scenario conjectured by D. Wolf (1991), in all known AKPZ
examples the function $v(\rho)$ giving the growth velocity as a function of the
slope $\rho$ has a Hessian with negative determinant ("AKPZ signature"). While
up to now negativity was verified model by model via explicit computations, in
this work we show that it actually has a simple geometric origin in the fact
that the hydrodynamic PDEs associated to these non-equilibrium growth models
preserves the Euler-Lagrange equations determining the macroscopic shapes of
certain equilibrium two-dimensional interface models. In the case of growth
processes defined via dynamics of dimer models on planar lattices, we further
prove that the preservation of the Euler-Lagrange equations is equivalent to
harmonicity of $v$ with respect to a natural complex structure.
