Bit of a conundrum this, could be as jimmy c says dirty tricks by the british government in colusion with loyalist interests to discredit SF/IRA and give an excuse to remove them from the political process.
But would the UK govt really want the return to the "armed struggle" that this would inevitably lead to?

The other scenario ,is that the IRA, realising that their objectives (united Ireland)were never going to come to fruition though the ballot box, pulled off the robbery leaving enough evidence to ensure that the UK government had no alternative but to expell them from the assembly.
This would ensure that SF/IRA could still portray themselves as the injured party and go back to "armed struggle "to deliver their objectives...Ake