I think it's his reward for being better. Here's my funny dick move story. One time my friend and I were playing a game with another friend online. We had him beat in 15-30 moves and it was clear that we were going to win. Then my friend starts quoting Ghandi and talking about the importance of non-violence. We then keep checking the King in hopes to capture him non violently until he finally took our queen with his Bishop. We started to argue about this events significance and relation to Pearl Harbor. But we decide instead of going after his major pieces to attack him pawns. Finally we get a message about 60 moves (?) in from our friend "why the fuck are you only going after the pawns?" to which we replied "Dresden yo' ." This madness goes on for sometime (with an epic battle for the last pawn) until we sack all our pieces for knights, the 4 horsemen of the apocalypse, and try to checkmate him with them until we stalemate. Directly after I changed my facebook picture to a motivational which read "the four horsemen of the apocalypse: Fuck you Matt." All the while the Dresden bombing was going on we were listening to Hanity and talking about justice and how accurately Hanity represented it and how killing civilians could be seen as an act of justice.
My point being, it's not bad sportsmanship. Some people are just fucking crazy.
I believe the NM coach said that to you for teaching purposes so u would develop an ability to fight and win even in bad positions and make ur opponent earn the wins or even better ur opponent stuffs up enough for u to win a game u should lose, and to teach u how stronger players finish u off so u can then learn from it and do the same when u get into such winning positions. But again that does not mean they have to finish it quickly for the reason i mentioned before. Keep learning and don't let little things like that annoy u it's just apart of the chess playing journey.