Down 15 you still need 8 + 7 to tie so you're going to need a 2 at some point anyway, it doesn't really matter when it is. Personally I always disagreed with the norm on that and would prefer to take it after TD #1 so I know right away if I need 1 more score or 2. There's no statistical advantage to waiting. I just don't agree with that strategy. The goal, if you are trailing in a game, is to eliminate a possession on every score if possible, especially this late in the game. Northwestern willingly chose not to do that in not electing to kick the extra point.