As I was reading through this forum a reply to that question by one of the forum's posters caught me by surprise. He related it the the ease of cyberbrain warfare, but in fact it has nothing to do with hacking. (At least in my interpretation, I understand it is never fully explained in the series.)
The method by which a stand-alone complex relates to cyber technology is in the transfer of information and synchronization of ideology. Simply put, the ease of data assimilation from cyber technology and ability, to, for example selectively rewrite one's own memories causes people to have not only similar data but reach the exactly the same conclusions about that data. This causes them to pursue highly similar courses of action. (e.g. the imitators who tried to kill Super-Intendent General Daido.)
The wiki article on GITS philosphy addresses the technological aspect with "It is presented as an emergent phenomenon catalyzed by parallelization of the human psyche through the cyberbrain networks. There is no original Laughing Man, no leader. Everyone is acting on his own, yet a coherent whole emerges."
A stand-alone complex is then a phenomena in which many actors reach the same conclusion and take the same actions individually, without communicating with each other. The complex aspect of it is that it has the appearance to outsiders of being a coordinated effort of a group, since the actions/motives/ideology appear to be so similar. Again from Wikipedia: "For example, a mere fire, no matter the number of deaths, is just a garden variety tragedy. However, if the right kind of people begin to believe it was arson, caused by deliberate action, the threat that more arsons will be committed increases drastically."
In fact, the Wikipedia article subsection on this aspect of GiTS is quite good, I wholeheartedly recommend it.