The blog posts, comments, and tweets aren't really what provided the direct pressure for him to resign.

Far more likely that corporate partners, donors, etc. called the remaining board members or anyone else of any influence and told them that the money would dry up if this problem didn't go away.

I find it interesting that the fact of his donation came up in 2012 when he was CTO of Mozilla (the organization he co-founded). This didn't become an issue in 2014 organically -- again I think it's far more likely that someone had it out for Eich, and saw their best opportunity to revive a two year old issue and get rid of him when he became CEO.