Hi, we live here in Argyle Park. No the Purple Line is safely (and I say this unfortunately as I am a big supporter of LRT transit and would love access to a quick car-less alternative to get to Bethesda) not going anywhere near us. There was some talk about Beltway widening.. but the pushback from the community has pretty much put a squash on that idea with the state (thus, the ICC was pushed through).
There have been some unfortunate circumstances for some of our neighbors (loss of spouse) for example. But If I were to guess - the economy has taken it toll on a lot of us and probably job loss. I think some of these homes were rentals and newbee landlords have decided to get out of the game. Go to a South Four Corner's Citizens Association Meeting for more intel. There is a real estate agent 'Tamara' something I think her name is... I see she has a lot of listings. She might give you some insights.
But the bottom line is we are a thriving and tight knit community. Crime is non existent, locality to everything you want could not be better. There is nothing to read into the for sales except this economy. We are not super rich folks and many homes are either long time (lived there forever) or first time home owners. Feel free to pick my head more if you want. Good luck.