The collective noun troop has been turned into a singular noun over the last few years by the news media ("Three US troops were killed today in Iraq."). The first few times I heard it used this way I wondered to myself, how many people constitutes a troop? but eventually I realized the way they are using the word the answer to that question is "one." My theory about why this has come about is that (a) they don't want to use soldier lest they offend the Marines and pilots, (b) they don't want to use men lest they offend the women, (c) they don't want to use people lest they offend the squeamish, and so they redefined troop from "a group of soldiers" (Merriam-Webster) to mean one individual soldier/sailor/pilot/Marine.
I think this usage goes back quite a bit further, perhaps even to the Vietnam War. Does anyone recall? However, my copy of the Associated Press Stylebook, vintage 1994, still defines "troop" as a group of soldiers. The word came from the Latin for "flock."
Anyway, I think Bruce has hit on several plausible explanations for the current usage. I lean toward the simplest—the need for a generic term for a soldier/sailor/Marine, especially for headline writers. When you have only a small space to get across the gist of a story, such a word is sorely needed.
I am just glad that the news readers have not landed on something worse. Given that our military is frequently referred to as "U.S. forces," it may be just a matter of time before we see a soldier called a "force."