  An upper bound on Lyapunov exponent of a thermal many body quantum system has
been conjectured recently. In this work, we attempt to achieve a physical
understanding of what prevents a system from violating this bound. To this end,
we propose - super-maximal chaos leads to instability. Our proposal is
supported by findings in a SYK lattice model, with a tuneable parameter, which
the Lyapunov spectrum depends upon. In the stable regime of this parameter,
along with incoherent metallic phase, the system exhibits another novel phase,
where transport is controlled neither by quasi-particles nor by diffusion. At
the phase transition, diffusion coefficient, butterfly velocity and Thouless
time diverges.
