I wonder. I'm leaning more towards there being aspects independent of Socionics to account for. I somewhat see theories such as enneagram, socionics, etc, as ways to interpret reality but not the answer to everything.
Was the assertive aspects of Se even originally mentioned by Jung? Even if it wasn't, I can see how Se can manifest as assertiveness. It makes sense that those who are well-grounded in reality and attracted to that which is tangible will probably be more likely to be more assertive or at least more confident in going after what they want and the confrontation that could result in that since they value and are aware of their environment as well as what's at their disposal. It makes sense that they are more likely to get aggressive since they have more territorial awareness and shit. As @silverchris9 said, I also think each type can be confrontational in their own way but maybe it isn't helpful saying that, haha.
I'd say SLEs are typically the most aggressive because of Se and Fi POLR. SEE have more diplomacy to their assertive nature. I dunno what would account for passive-aggressiveness.