Oh, well, that is easy. The reason, why it is the last song before you left the house, is based on the very nature of earworms. Earworms are veeery tiny creatures whose size is the direct opposite to their ego. Usually it sits right at the brink of megalomania, let them constantly feel the urge for world domination. So the first thing they do when born at the speakers is riding instinctively on the sound waves to the next exit to infest as many brains as possible. So when you leave the house, there are already gazillions of earworms there waiting to attack your brain and use you for transport to other brains (since the sound waves already died down when you switched off the music to leave).
Of course each song creates a different tribe of earworms, all of the magalomaniac world leading kind. Therefore song after song there are epic battles at the door for strategic exit domination. Unfortunately for the tribe already sitting there, the newly arriving worms are mercilessly clashing onto them with a whopping speed of about Mach 1. Only few of the strongest worms are able to survive that…

I guess that makes it pretty clear. By the way, funny and nonetheless informative blog you have here! It is my first visit, I have got the link from my friend Anja over at Planet Cola.
Keep schlumping,
Bob