In "A & P," Sammy stands up for Queenie in attempts to be romantic and heroic. Queenie and the two girls walk into the A & P wearing only bathing suits. They are flouting conventions, being relatively rebellious; relative to social mores of the time (1950s to the early 1960s). While Sammy's intention is based on a romantic/heroic notion, his gesture is also a rebellious one since he stands up for the girls (who are the initial rebels in this story).
Sammy also recognizes that he's still young enough to rebel. He understands the danger of becoming like Lengel. He understands that his life working at the A & P has begun to condition him. He refers to the customers as sheep. He describes the general atmosphere as if the customers and workers were zombies:
I bet you could set off dynamite in an A & P and the people would by and large keep reaching and checking oatmeal off their lists and muttering "Let me see, there was a third thing, began with A, asparagus, no, ah, yes, applesauce!" or whatever it is they do mutter.
Sammy is restless. His decision to quit is spontaneous but he's recognized that he'd begun his adult life with a redundant, thoughtless job and he doesn't want this to become a habit.
After quitting, Sammy considers his decision to quit. He's proud but also anxious. He thinks, "my stomach kind of fell as I felt how hard the world was going to be to me hereafter." This implies that Sammy believes he will make be making more difficult, rebellious, nontraditional choices in the future.
Given Sammy's feeling that his future will be rebellious and given the time period he's living in, it would be very possible that Sammy would embrace the Civil Rights Movement, particularly during the late 1960s. This movement includes protests in support of rights for women and minorities, anti-war demonstrations, and other anti-establishment ideologies. In other words, this initial rebellious move might lead him to consider the wider implications of social rebellion on a national level.