The threats is based around a website, claims that Swalwell has ties to housing developers Swalwell has been facing this complaint about his support for a local project for some time (though he was only elected in 2010).
More interesting is that the article questions whether it is as "AstroTurf" group looking to help Stark in the election. Under this theory, the recall threat operates to muddy the waters against Swalwell and create the appearance of a grassroots organization rising against Swalwell. At the very least, it continually reminds voters that Swalwell has supported a seemingly unpopular decision. None of this is to say the astroturf claim is accurate, but it is being bandied about. Off the top of my head, I don't remember any prior "astroturf" recall threats, though I'm sure they are out there.