but where was the imminent threat?
Two suspicious-acting characters amble up, block the main exit, take on dominating roles, express their intent to rob, begin terrorizing the sheeple, and make it clear they are in charge and that the customers will obey.
Ability: two healthy males. They may be outnumbered, but they can cause grave harm bare-handed. Anyone that challenges them can be "disposed of" very quickly. While no weapons shown, behavior exhibits confidence that they have the means to cause grave harm.
Opportunity: physically right there, entrance dominated. Crowd is self-limiting for movement. Cashier has limited movement behind counter. Almost nobody there would have thought of the back exit had sm not mentioned it. If capable of causing harm, they sure had the opportunity to cause it.
Jeopardy: overt expressions of dominance, intent to rob, and thinly-veiled expressions of "comply or suffer". These guys were not shoppers, they were plainly there to cause trouble with an easy segway (sp?) into violence if anyone crossed them. Sometimes, you just know - and everybody there, sm, anonymous helpers, customers, and clerk, KNEW.
Ability + Opportunity + Jeopardy = seriously bad situation.
Yes, it's not as overt as "they ran in shooting randomly".
The real world is often not that easy to articulate in stark black-and-white terms.
Streetwise people will recognize a developing "situation" long before it reaches clinical clarity.
By the time a lethal situation DOES reach total stark clarity, it may be too late.
Our heros recognized what was developing, took action, controlled the situation, and gave an excellent subtle-but-clear response: "don't go there".