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.
I was reading the page looking for grandparent's question, and I still don't see why bbm would be such a huge feature that bb owners just can't live without. Sure, I can see how it may beat sms where international communication may be concerned, and texting someone in a different country doesn't work, and I can see how the group chat is useful, but every single other IM client can offer the same thing... I just don't get it.
Ted Livingston worked for RIM and saw problem, a big gap that needed to be filled in. Which was, taking the best black berry app and making it accessible on every major smart phone out there.
Also they didn't just kick them out of the store, but revoked PUSH and signature keys, which is pretty a drastic move on their side. That makes me certain there is more to it.
Other IMs are very, very crummy experiences compared to BBM between BB devices. Kik is the first one that may be able to compete with and replace BBM (if only because it took a lot of "inspiration" from BBM), which is a big danger for BB as BBM is the only strength they have in the consumer market (in the US a blackberry is a corp phone, but in Europe it's used a lot by high-texting-volume teenagers and young adults)