So, as of update 21 Quick Thinking causes constant lethal stagger locks from even weak sources of damage, including DoT effects.
At first glance this looks like a reversal of the fixes to Quick Thinking made in update 11 back in 2013. This when the Rage/QT combo was still really strong (for almost 2 years after), and when enemy levels in common missions where much, much lower.
The threshold for a stagger now appears to be as low as ~50 damage. Even a level 20 sapping osprey can stagger