Now I don't know how much this plays a part but a few weeks back CEO John Chen had a public spat with the carrier about a ad it sent out via email. T Mobile semi apologized and did a promo it backfired for BB(of course it would) 

So this news means that BB was still butt hurt and decided to part ways but honestly it's going to hurt them. Doing something like this when your company clearly has no direction shows other companies you are unstable. Honestly I feel like it's a matter of time before Verizon, Sprint, or ATT follows suit or all three at once.