I'm fairly certain it has nothing to do with kills in a round, map, series of maps, or your overall K/D. Today, one of our members had this message pop up. He was playing well but he wasn't on any sort of killing rampage.
During a run of good luck tonight I went 10-0 in a couple of rounds and had some nice kill totals for the entire map. Over that run of luck I went from level 23 to 26. I never got the message. If it's linked to something in leveling or k:d, I haven't seen it yet and I'm sure I won't luck into better rounds than tonight.