Ann: Race Leadership Changes, page-155

My take on Dr T's exit
Assumption 1: As the largest shareholder, Dr T has his financial interests (and that of the shareholders) as his first priority. His position is too large to sell down a significant amount without any major liquidity events.
Assumption 2: Dr T is an intelligent investor - he is unlikely to let "serious differences" in opinion cause him to take action which could lose him many millions of dollars.

Given his exposure to RAC, even in the case of disagreement it would seemingly make fiscal sense for Dr T to remain as part of the company (perhaps on the board but not as CSO) so he can argue in favour of his opinion forward for the companyWith this in mind, I find it reasonable to suggest that Dr T sees his best way to maximise his investment in RAC is off the board entirely and as a shareholder. In this position he is able to disagree with the board of RAC (such as in the case of a buyout offer), and as the largest shareholder, and as an individual who is held in high regard by many shareholders (undoubtedly many large shareholders) he has great power to influence the direction of the company. As stated in the announcement and repeated several times in the video interview, Dr T is confident in the competency of the team in executing the current and upcoming trials and plans of RAC - this is compatible with a situation where Dr T feels he has greater potential to maximise his investment value as a shareholder and off the board.I think it is possible, especially with DCB's international networking, there have been tensions that have grown around what is fair value for the company, and this has acted as an impetus for Dr T's exit.

In a bit of left field speculation, Dr T's exit may have been somewhat spur of the moment upon catching wind of an upcoming buyout (hence the suddenness of the trading halt).