I'm not really sure either but I believe I've gotten it for simply killing 2 enemies within a short amount of time of each other. Not sure if it's that critieria or something else.
Yeah, two guys down in a short time frame, I get it all the time. It's weird that it's the only killstreak message we have though.. It isn't that difficult to take out two enemies with gunplay and it's very common to do it with nail bombs and mollys. It should have been something like 3 kills in a short time is a rampage and 4 gives an executioner bonus.