There's actually a fair bit of theory on this. For example, Bourdieu's book Distinction often drifts into asides about table manners and other related habits as things that signal and situate the classes. It isn't that all table manners are bourgeois, but that what are table manners tends to be different according to one's position in the social order.

If you don't think table manners are bourgeois then you never really learned table manners

The elbows on the table thing, a more guarded way of eating, is associated with the lower class. From sailors to factory workers to the the homeless to miners to prisoners etc etc, who usually ate in cramped spaces shoulder to shoulder and/or are guarding their food would carry on this habit outside of said environment and the bourgeoisie didn't want to be associated with such behaviour.