A couple reasons spring to mind:

1) Hive Mentality. Most people just follow each other around and so if one yolo-loving-member decides to pick a fight, most likely a few others will follow. Then other alliances will see the that's where their team is heading and follow like lemmings.

2) Kills. There might be a point when you know that your team just isn't capable of winning the match - either you're trailing too far behind or simply lacking the skill to keep up. Might as well start picking off the easiest to kill team and get a few achievement points. That 'Hand of Mercy' title isn't going to earn itself.

3) Most people are new and don't understand strategy. All they know is it's PVP and think the strategy is to kill other people. Points? Ice? Wins? Losses? No clue on those things.