Alternatively, you may be singing without quite realising it. A tune has got into your head and won't leave until you've sung the same few lines over and over. Much to the annoyance, most likely, of those around you.
I tend to fall into this category, I have to admit. Often, I'll start a tune and then tail off, having been distracted by something else - otherwise, I'd have gone through the whole song, knowing the full lyrics.
Julie is also a subconscious singer, and it's usually highly entertaining, because knowledge of actual lyrics isn't necessarily essential; especially since it's all rather stream of consciousness stuff.
Normally, this isn't too distressing, as Julie will have just thought of something that reminded her of the words from a particular song, and she will simply riff off that. However, once in a while, Julie just begins singing. No external input, nothing on her mind - and no restraint...
Wait, so that's two possible songs, two bands, a traditional location for sing-songs and a WTF.
Now that's a nasty mash-up.
Let's have ourselves an alternative mash-up of the odd persuasion...