I think Wolfgang's probably nearest the truth. There are plenty of hardliners who have no truck for Adams and the Good Friday process, and the republican movement is in the process of splitting all the time.
The Contunuity IRA, the Real IRA (thought to be behind the Omagh bomb) or some other independent faction could well have carried out the robbery, and I would imagine that Sinn Feinn and what's left of PIRA are spitting nails about it, because it plays into the hands of the die-hard 'no surrender' lunatics who will blame anything and everything on SF - much like the way the PLO gets blamed by Israeli hardliners for the actions of Hamas and Hezbollah on the West Bank and in the Gaza Strip.