I too have seen this at Chasunas where the Chosson is from Hungarian stock. Very curious because no one provided me an answer when I asked around. What is more curious is that when a kever of someone from Moroccan stock is visited (after the week, shloshim, year), a relative of the deceased spreads a raincoat (though not exclusively black) over the grave.
A chosen wears a kittel because it is a yom kipper for the chosen and kallah however he puts a coat on top to hid the kittel from the public so as not to put the yom kipper on them it is a personal yom kipper (the daven mincha yom kipper)..
I was told by a litvak that the Hungarian minhag is because of outdoor chuppos. He wears a kittel because it’s a personal yom kippur for him. He wears the raincoat to keep the kittel dry.