[Physicist]

Another possibility is that this was not an assassination attempt, but that the allegation of one is being used as a pretext for a purge.

[JasonC]

Indeed, much more likely. As for the cell connection, the obvious rationale is that they suspect somebody was listening. That is, they were moving missiles or whatever, and people involved in the shipment communicated by cell. The NKs then think, "ah, so the NSA must have heard them". Sounds like the NKs think it was well aimed covert ops. Doesn't mean it was, incidentally. Paranoids can think whatever they like - even when somebody is out to get them.