  Electronic structure calculations for spinel vanadate ZnV$_2$O$_4$ show that
partial electronic delocalization in this system leads to structural
instabilities. These are a consequence of the proximity to the
itinerant-electron boundary, not being related to orbital ordering. We discuss
how this mechanism naturally couples charge and lattice degrees of freedom in
magnetic insulators close to such a crossover. For the case of ZnV$_2$O$_4$,
this leads to the formation of V-V dimers along the [011] and [101] directions
that readily accounts for the intriguing magnetic structure of ZnV$_2$O$_4$.
