i think some of it *is* related. because they will do it all over again when they haven't interacted/seen each other for a while. even if they live together or, for example, first see each other at a community lot and meet again two minutes later.
this hello-before-interacting thing (*and* the greetings that don't "match" the relationship) has been going on for quite a while, actually.
and i agree that the kiss/hug/wave/shake hands thing happens in-between tasks (like eating or watching TV, as you mentioned). that is, IMO, the excessive part.
so i'd say that it's a mix of a filler action with, like you said, some sort of... socialising malfunction. 😉
BTW, also related to these interactions/greetings that don't make sense: i once wrote about the kissing when Sims dislike each other (or are distant/just friends/have an empty romance bar/etc.), but i was told it belonged in feedback.