Adding une fois at the end of a sentence is a common way for French to make fun at Belgians because it is a stereotypical ending. Doing it became extremely popular during the seventies and eighties because of some of Coluche's sketches and jokes that were sometimes taken too literally.
Of course, the Belgians are not happy when people make fun or their accent, a deplorable French habit that was recently given a name, glottophobie, and also when people make them look like idiots, but who would be?
This is surely even more annoying since this une fois is not even common among the majority of French-speaking Belgians (Walloon), but rather a characteristic of Dutch speakers (from Flanders) when they speak French or from French speakers from the Bruxelles region, under the influence of the eens (i.e. once) that Dutch sometimes insert into to their sentences. See this page for more details.
Note that it is not just using une fois and mimicking the Belgian accent that made this inappropriate, that's the accumulation of histoires belges (Belgian jokes) that not only made fun of the accent but were also quite disrespectful towards Belgians if taken litteraly. While self-mockery is doable in small doses, it can't be the only topic of every conversation referring to Belgians. It might be compared to blonde stories. If you are a blonde girl and the only thing you hear about you are blonde jokes, you surely aren't going to be happy.
