Is there anything suggesting it's SourceForge itself doing this and not just (an improbably widespread, admittedly) set of account breaches? It makes sense -- acquire accounts, enable ads, profit.
Why are we doing this? We want the SourceForge software directory to be as useful as possible. When you come here to search for a piece of software, we want you to be able to find it, and find the most up to date releases. And if that software isn't hosted on SourceForge, we still want you to be able to find it. Or if a SourceForge project has been abandoned, we want it moved to the mirror and maintained, so you can always find the newest releases. Millions of people use SourceForge every day to search for Open Source software, and we want to give them the best experience possible, even if the best answer to their search is a project hosted elsewhere, or an abandoned project newly maintained by the SourceForge team.