No need to explain the story itself if the song itself does that. We had one woman who was singing Nancy Whisky and in between each verse explained what was going to happen in the next verse! It was quite frankly a tad annoying though no-one said anything. However surely nothing wrong with explaining the background to a song before singing it? As long as it isn't too long winded and is interesting I think that often enhances it. I don't think there is any excuse for publicly barracking someone though. A wee word in the ear is surely preferable?