Twitter’s original outside investors, Fred Wilson of Union Square Venture and Bijan Sabet of Spark Capital, have left the Twitter Board, reports reports Peter Kafka of allthingD.

According to SAI:

“there is a schism on Twitter’s board. It sounds like a battle between the old guard and the new. Specifically, sources chatty with Twitter board members say that the company’s original venture capitalists, Fred Wilson of Union Square Ventures and Bijan Sabet of Spark Capital, are being increasingly isolated – edged out, even.

Why? People speculate that Jack Dorsey’s return to Twitter has something to do with it. Remember, Jack was Twitter’s first CEO before Wilson pushed then Twitter chairman Ev Williams to push Dorsey out. (Long blamed for this coup, Williams says “the VCs asked me to do it” in the Fortune story.)”

Another theory: Fred and Bijan are early stage investors who sit on a lot of startup boards. It’s possible they (or Twitter’s other board members) felt that they didn’t have as much to offer Twitter at this stage.

Kafka says USV and Spark still own a majority of their Twitter stakes.

We can’t imagine Fred or Bijan would WANT to leave Twitter’s board. Twitter is going to grow into a big company, and it’s always helpful for VCs to have a seat on the boards of companies that could acquire other startups in their portfolios.  One of Google’s VCs, Michael Mortiz of Sequoia, didn’t step down from its board until 2007.  It’s other big VC investor, John Doerr from Kleiner Perkins, is still on the board.

All that said, it’s very hard to fire board members. It’s possible that sitting on Twitter’s board simply grew into an unpleasant experience for Fred and Bijan. Twitter is now a huge company. It’s in California. It is a pain in the ass to sit on boards.

If there was friction and Fred and Bijan were frustrated, they might have just said enough is enough.