My theory is that you have sent a mail to a contact who uses Outlook. They have saved your address in their address book. The address book has subsequently been harvested by a bot and your email address sent on to a spam list.
I am sure that is the reason. I don't see how anybody (robot or human) could extract somebody's email address from pprune.
For what it's worth, I get zero spam to my pprune address, but get thousands of spams per day to other aliases on that domain.