If Senators are making coordinated calls for Franken to resign and his office has scheduled ...

an announcement for tomorrow.

Then the odds are that his decision was made prior to any tweet from any senator. Franken can see the political calculus on the wall as well as anyone else, and he knows that the steady stream of accusers (be they real or fake) is a distraction that no one needs or wants.

Franken isn't being forced out by a surprising onslaught of calls to resign. He's taking a hit and allowing his caucus to take cover beforehand. This was queued up and ready to go.