Interesting that it seems to have harvested an address book or have access to your contact information.
Have you uploaded your contacts to a third party such as Google or a social network app?
The facebook app is the only app that harvests the Address book on the Blackberry as far as i know I am probably mistaken but its a srtart
Was this a one time thing or are your contacts still getting the messages? If they continue to get them, turn your phone off for a bit and see if it continues. If it still happens when your phone is off then it's not from your phone.