Revised short answer
With base open Icbo flows thru emitter with current gain until Vcb max is reached then it becomes a zener then if Rc is low like a voltage source, leads to thermal runaway unless there is current limiting to prevent PD and temperature rise.
As I recall, the normal diode reverse bias leakage on the collector is on the order of 1 uA and roughly doubles every 10'C rise.
If I understand it right, Icbo is measured with the emitter open and Iceo is measured with the base open which creates some equivalent R'ce. Then under the conditions of high Vce across this leakage resistance can cause some self heating.
Now if the collector and emitter R's are much smaller than this equivalent Rce leakage then Vce does not drop much due to leakage. Thus the leakage current creates more rise in Pd in the transistor than the external resistors.
I think the runaway condition comes when the transistor is more thermal insulated and higher Vce such that the leakage current is a significant source of heat, which does not conduct externally.
I have never personally encounter this problem as I recall because I never used very high Vce thermally insulated parts that cause the temperature to rise 10'C from twice the leakage current.
Normally H biased parts are very stable and only drift very little with temp rise unless you have very high DC gain, but not to the point of thermal runaway.
What's your experience?
Now look at a photo transistor with no input current. These are designed for high current gain but very low leakage of Iceo and there is no base bias current. (Iceo means base open)
Iceo= 0.1 μA @ Vce=20V where (β+1)Icbo=Iceo thus leakage R'ce = 20V/0.1 μA = 200 MΩ @ 20V but BVceo = 60V min @ 0.1mA then R'ce = 60V/100μA drops to 0.6 MΩ the threshold of avalanche condition.
Thus if Rc was say 10kΩ much lower than R'ce, Pd= 60V* 0.1mA = 6mW the opto would be quickly followed by thermal runaway if Vce exceeds 60V. ( depending on tolerance and process quality of the part.)
I hope this answers every question, otherwise feel free to ask more.