It mostly happens when I'm stressed and driving with the radio off. I'll just be minding my own business and BAM! I'm 5 minutes into a scat singing solo which lasts until I reach my destination. It doesn't matter how long the journey is, I've stretched it for over an hour wile stuck in traffic once. Sometimes it's a lilting melodic solo, sometimes it's 30 minutes of unhinged post bop madness.