A software issue... Hm, lets see about some scenarios.
a) Could it be that the system got encrypted and a note was left where to pay the ransom? The module did not have a Russian keyboard attached to prevent the pawning of the system. The thrusters simply fired as a proof of encryption. Payment was probably sent fairly quickly.
b) The software malfunction was a spontaneous act of self-awareness of the module. There is no point in being hooked up to the ISS and live out a life locked in one place. The module decided to act and tried to escape.
c) Is was a rather boring lapse in Q&A. They only simulated probable error scenarios and expected most stuff to "do well". Then, thruster-fire-hell happens when some shit hit the (vacuum) fan igniting the thrusters.
d) They simply ran out of luck.