How is the rate in which NPCs drop cargo determined?

I have been pirating NPCs in ED lately, trying to figure them out completely, what works, what doesn't. I've noticed that while flying a Diamondback Scout/Viper/Cobra, Haulers tend to drop cargo more often then not when their hulls hit a certain threshold, usually 40% hull remaining. As the ship gets larger, their tendency to drop cargo plummets, going from Hauler-Cobra-T6-T7-T9.

Is this determined by ship size? Elite combat rank? Is an outcome generated before I even deploy my hardpoints?

This is troubling because every trader not flying a hauler would normally allow themselves to be killed, rather than dropping even their worst cargo. All I'd like is an answer as to how this is determined. Should I not be gunning for any ship better than competent, my current rank?
 
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