1. They have a serious legal or ethical concern about some activity of their company, and feel it can only be redressed with the help of the media. Karen Silkwood comes to mind.
2. They are happy people who want to share their happiness with the world. They think something is funny and want to enjoy it with the rest of the blogosphere, because they believe it will do no harm.
3. They don't really feel like they belong to the company that issues their paycheck, but are rather members of a larger community they like much better. This is particularly true of journalists who leak to other journalists to demonstrate that they are all part of the same great Big J team.
4. They are about to leave the company and are looking to suck up to the guys who are going to be covering their departure or their arrival at their next career pit stop.
5. They are powerless little weenies who want to appear more important than they are for the five minutes they are at the center of the information transaction.
6. They are has-beens or wannabes who don't want to admit to the blogger/journalist/aggregator that they don't know anything about the topic under discussion. Since virtually anything, no matter how blind or unsourced, is now fit for distribution to the Twitterati, this is a glorious new day for such people.
7. They are malevolent saboteurs who hate the company, hate management, hate anybody who is doing better than they are at the corporation. They are always looking to skewer their enemies and make them look bad in the public eye. They view leakage as a way to undermine everybody whom they despise. A day without a leak that hurts somebody is a day without sunshine to them. They sit in their little den afterwards and cackle at the malicious carnage they have wrought.
8. They are to be forgiven, for they know not what they do.
9. Everybody is doing it.
10. They are only following orders.