I can only speculate on the reasoning, but Kik competes with BlackBerry Messenger, which is one of the key features that keeps Blackberry competitive in the smartphone market. Its exclusivity to Blackberry devices gives them some network-effect lock-in to the vendor and it is the best messaging system out there.

A lot of their marketing is focusing on how great BBM is and how buying a Blackberry gives you access to this exclusive network of people. BBM is also the source of all those issues in UAE and India where its so secure that governments were threatening to block Blackberry because they couldn't snoop on BBM messages. Basically, BBM is the killer app on Blackberry.

Kik would commoditize BBM across any platform allowing users to freely move between them. The literature for Kik itself describes it as a BBM for all platforms, and I can see how RIM would be concerned that they'd lose one of their few advantages in the smartphone market if Kik supplanted BBM. Again, not sure if this actually was the "behind the scenes" cause, but RIM would definitely have a motivation to kill Kik.