  Some low-$n$ kink-ballooning modes not far from marginal stability are shown
to exhibit a bifurcation between two very distinct nonlinear paths that depends
sensitively on the background transport levels and linear perturbation
amplitudes. The particular instability studied in this work is an $n=1$ mode
dominated by an $m/n=2/1$ component. It is driven by a large pressure gradient
in weak magnetic shear and can appear in various high-$\beta,$ hybrid/advanced
scenarios. Here it is investigated in reversed shear equilibria where the
region around the safety-factor minimum provides favorable conditions. For a
certain range of parameters, a relatively benign path results in a saturated
"long-lived mode" (LLM) that causes little confinement degradation. At the
other extreme, the quadrupole geometry of the $2/1$ perturbed pressure field
evolves into a ballooning finger that subsequently transitions from exponential
to explosive growth. The finger eventually leads to a fast disruption with
precursors too short for any mitigation effort. Interestingly, the saturated
LLM state is found to be metastable, it also can be driven explosively unstable
by finite-amplitude perturbations. Similarities to some high-$\beta$
disruptions in reversed-shear discharges are discussed.
