Maybe not because of the violence itself, but also because of darker themes in the plot or the more modern settings. I'm mostly thinking of the ending to Modern Warfare 1 especially.
Perhaps, but they've always been war games with lots of death, and gruesome death scenes. Not as "realistic" looking as the CoD4 ending, but still there.
Looking over my games, most of them are rated M, except Batman Arkham City/Asylum and a Metal Gear Solid one. The Metal Gear Solid came with a trilogy (HD collection), but the other two are rated M. I've played through them (not completed), but it didn't look like much changed gameplay wise.
Where do they draw the line between T and M in these games? I'm not talking about GTA or other obvious games similar that obviously deserve an M rating, but the ones that had T rated prequels.