SPOILER: Stan's mom and dad divorced because Randy decided he liked "Tween Wave" (the "music of the period between 2009 and 2013") and started playing at a bar at night, where he called himself Steamy Ray Nicks and found someone named Steamy Vaughn (or maybe he's Steamy Ray Vaughn...I forget, anyways) who is in the same position as he is in. Sharron sees them and decides that they aren't good together anymore, but she gets custody of Stan (apparently) and they move somewhere greener.
Also, Stan is diagnosed with "cynicness" and starts to feel that everything in the world is a turd. He complains about it all the time so Kyle, Kenny, and Cartman stop hanging out with him.