As I always understood it, an apostate was someone who opposed the organization. Hence why they were referred to as 'opposers' by the WTS. If you went against the direction of the GB, you were an apostate. This could be as simple as disagreeing with an interpretation of Bible texts or differing on doctrines and policies. As joey jojo said, they feel that the GB stands between Jehovah and the rank-and-file. To disagree with the GB is to oppose Jehovah.
And let's be clear on CSA- they only treat it so seriously because they have been forced to, both by cultural and legal pressures. Before, they may have claimed to take it seriously, but their actions didn't show that at all. They've been shamed by 'the worldly' into doing something they should have been doing from the beginning, if they were indeed 'spirit-guided.'