Sigh... Contrails 'spread' the way they do for reasons similar to the supercooled water trick that everyone is doing nowadays, but in reverse. the passing jet disrupts normal temperatures and pressure way up in the atmosphere, resulting in condensation, which cascades outward for a while, then spreads out as it dissipates and returns to normal.
The difference in temperature and pressure is key - Its why airports aren't covered in a constant fog. Its also the reason why this trick works as it does.
[youtube=560,315]Fot3m7kyLn4[/youtube]
There is as much 'woo' about this as there is about contrails.