While the intent of SOPA is to halt internet piracy by holding content hosters (rather than just uploaders) culpable for copyright infringement, the members of NetCoalition are concerned that it would fundamentally change the way our internet functions. Any kind of content hosting site (such as YouTube) would have a very difficult time remaining in operation under the new legislation, as they would be violating copyright law whenever an infringing file is placed on their site. This means either constant legal battles, extremely broad licensing agreements, or company review of any content that would go on their network.
While NetCoalition would probably like to avoid a service blackout, since they would take some backlash themselves, this is one method that the supporters of SOPA simply can't match. Should such a blackout occur, you can rest assured that you will hear about it on Ology first - probably because the rest of the web is offline.