  Calcium vanadate CaV$_2$O$_4$ has a crystal structure of
quasi-one-dimensional zigzag chains composed of orbital-active V$^{3+}$ ions
and undergoes successive structural and antiferromagnetic phase transitions at
$T_s\sim 140$ K and $T_N \sim 70$ K, respectively. We perform ultrasound
velocity measurements on a single crystal of CaV$_2$O$_4$. The temperature
dependence of its shear elastic moduli exhibits huge Curie-type softening upon
cooling that emerges above and below $T_s$ depending on the elastic mode. The
softening above $T_s$ suggests the presence of either onsite Jahn-Teller-type
or intersite ferro-type orbital fluctuations in the two inequivalent V$^{3+}$
zigzag chains. The softening below $T_s$ suggests the occurrence of a
dimensional spin-state crossover, from quasi-one to three, that is driven by
the spin-lattice coupling along the inter-zigzag-chain orthogonal direction.
The successive emergence of the orbital- and spin-driven lattice instabilities
above and below $T_s$, respectively, is unique to the orbital-spin zigzag chain
system of CaV$_2$O$_4$.
