If SM was discussing an upcoming wedding with a friend, naturally discussions surrounding relationships, love, etc. may come up. If SM expressed unflattering commentary about BM, perhaps this upset him...
Another idea is that perhaps SM expected a certain level of treatment for Mothers Day, but BM disagreed, thinking he is the breadwinner, but "she is only a stay at home mom". This could be the source of an argument.
Or perhaps the move to Colorado continued to be a source of tension. Perhaps he felt she wasn't appreciative of their expensive home and his hard work.
Any number of things can spark an argument between a couple.
Of course LE know the content of any messages with the friend, which might provide some clue.