"On Monday morning Elon Musk filed a Schedule 13G disclosing that he owned 9.2% of Twitter Inc., which was a fun surprise. Part of the fun was that a 13G is a form that is used by passive investors who do not plan to meddle with the company’s management, and that doesn’t sound like Elon Musk. Activist investors are supposed to use the more detailed Schedule 13D. But Musk checked the passive investor box.
Part of the fun was that Musk’s 13G seemed to be late? It seemed to say that Musk acquired 5% of Twitter’s stock on March 14, and under the rules he would have 10 calendar days — until March 24 — to file the 13G (or 13D). But he filed it on April 4, 11 days late.
On Tuesday afternoon Elon Musk filed a Schedule 13D disclosing that he owns 9.1% of Twitter. So! This does not necessarily prove that he should have filed a 13D in the first place, or that his intentions were activist-y all along. Between Monday morning and Tuesday afternoon, various things changed."
Based off the above, the interest in joining the board could have been a clever way to right-size, is the fun conspiracy theory at least.