A Cease & Desist would be a pretty good explanation. Part of that process is showing that you respected the owners' rights, etc. Taking down the whole site if just one publisher went after them is unlikely.. seems like there's something much bigger here.
Maybe a few publishers all attacking at the same time? Getting buried in legal isn't fun.. Or maybe not enough revenue? Or maybe a buyout or...? No inside info here.
I don't see how a C&D could possibly achieve this unless GOG was too broke to afford any legal representation at all. Even the scariest C&D letter from a single publisher shouldn't be enough to suspend GOG's agreements with other publishers, and for all the publishers to collude to shut down GOG wouldn't make much sense (why did they make agreements with GOG in the first place?), and ought to run afoul of antitrust laws anyways.
Maybe a few publishers all attacking at the same time? Getting buried in legal isn't fun.. Or maybe not enough revenue? Or maybe a buyout or...? No inside info here.