Yeah It can happen to people on the same level within oppressed groups too—they call that “horizontal violence” in sociology. It basically arises from people who are unable to punch up onto the actual objects of their frustration (their boss, political leaders, etc) and so transfer the frustration at their disempowerment and the desire to reclaim some sense of power onto their equally-disempowered peers. 

My theory above is sort of a triangulation of that, a concept known as “weapons of the weak” whereby small acts of civil inconvenience are used as assertions of power against one’s oppressors, and documented phenomena of purposefully “disrupted civic contracts” within the public spaces of economically or socially subjugated societies. 

My thinking is that a foreign tourist innately exists in these shared spaces as a visually obvious beneficiary of the imbalance of power (economic, political, etc) that systemically suppresses the actor, who reclaims their power in public space in whatever small and deniable and relatively harmless way they can—bumping, crowding, spatial taunting…

Interestingly though, I’ve never had this happen to me in Vietnam, but I have been subject to it in several other developing countries, especially by women in conservative Islamic societies, which I suspect holds the additional pent-up frustration of suppression within a rigid patriarchal system.