Pyroclastic flows and snow avalanches sometimes exhibit a rapid deceleration of their dense flow fronts and detachment of their dilute clouds. This behavior is also inferred for submarine flows and could explain stepped thickness patterns in their deposits. A similar abrupt transition process occurs in particle-laden, lock release laboratory currents with relatively high concentrations. New experiments on nonparticulate, solute-driven density currents were run to investigate the cause of abrupt transitions. Abrupt transitions occur in laboratory currents with Reynolds numbers (Re) less than 1000 and are interpreted, supported by theoretical scaling analysis, to signify a change in dynamic regime. Currents with high Re, which do not show abrupt transitions, undergo a downstream change in dynamic regime from (1) inertial slumping to (2) inertial-buoyancy spreading to (3) viscous-buoyancy spreading. In low Re currents that undergo abrupt transitions, however, the duration of the second regime is very short, and hence they appear to pass directly from the quickly moving slumping phase into the slowly moving viscous phase. Scaling analysis indicates that an abrupt transition should occur in currents below a critical value of Re of ~10--5000 for currents with different initial aspect ratios. Given that natural flows typically have greater Reynolds numbers, we suggest that abrupt transitions in laboratory and natural currents are likely to be dynamically different. This work has important implications for the physical modeling of gravity flows.