They fear that they will be disconnected from the network before their fork happens and don't want that to happen.
And could you tell me/us why would they get disconnected?
To be honest I don't quite get it, what is to gain here with this practice? (lets assume its not privacy)
@gmaxwell you seem like a competent person, could you explain with simple language and easy allegory, what is all this fuss about?
They would get disconnected because most nodes are Bitcoin Core nodes and as far as I know Bitcoin Core nodes will disconnect from the S2X nodes.
They just don't want to their client to have very few connections (some might possibly have trouble connecting at all), so for the purpose of their client running just as well as Bitcoin Core until the fork, they did this so that Core nodes won't discriminate against them. However, this would pose bigger problems during the fork as these clients will get disconnected from Core nodes anyway as they follow a different blockchain and basically a different network. S2X devs might not care so much for that as they maybe just hope that they will convince Core users to switch by then or it might be financial reasons as the slower client with fewer connections might discourage S2X supporters to keep supporting them.