There is a ranking reason to do this. You do gain more/ loose less points the higher your position. You could make a steady path up the ladder by coming 2nd every game. So in many ways securing second place is a viable ladder strategy. This is especially true when it is a 4-6 player game, there is a lot of points given for coming 2nd in a 6 player match if it is balanced in player ranks, there is also an acceptable number given for 3rd.
If you know that you have no chance of beating the dominating player, it's better to destroy the other opponent. A lot of times, the stronger player will finish him off if he's the faster one to kill or has the most cards, etc... Sometimes I'm nice and blow away the other players before I kill the one that "helped me."
I've also been in situations where there's 5-6 players and I'm doomed because I got gangbanged, If I have 3 cards and can trade I will do so to bring myself to zero cards and then never attack anyone again. It's a guarantee to 2nd place since there is absolutely no benefit in killing me other than to finish the game. However, I can imagine there are scenarios where you may have pissed someone off so much that they will kill you regardless of whether it's beneficial or not. I don't usually end up in these situations since I only attack to kill or collect an easy card. You know it's a skilled player when someone tries to destroy him but fails and instead of retaliating, he uses his card trade to go kill someone else who will benefit him. :)
I've also been in scenarios where two weak players gang up on me, gain some ground and then they end up fighting each other when I deem myself dead. Why don't they kill me? I dont know. But the one that thinks I'm on his side for whatever reason is who I kill off as soon as both of us have cards so I can win the game.
So in a nutshell, 2nd place is a big deal and players will try for it if they are doomed or the risk is too high to attempt at taking 1st.
Sounds like a good strategy for a fixed card trade in game. I don't touch anyone unless I'm going to kill them for cards. It's all about making them feel like they are not threatened and let them trust you and focus on other people with the little pows and runs here and there (weakening each other). Then once strategically placed, kill all of them in one turn and go to another game. Armies and cards are all that matter in games with lots of players. It's all political for the longer, low player games. Attacking someone without killing them means there's a chance of triggering emotions for a counter-attack. Even if someone hits some of my armies like you describe, I won't react.