That's not unusual at all. A lecture at Harvard Business School about startup IPO mentioned that first time founder CEO and first time board member get more often then not replaced. Read about the business case of Lewis Cirne who got replaced at his first company Wily, and founded later New Relic with many lessons learned.
"Get more often then not replaced" not quite sure what you meant here. Could you say a bit more?
Agreed, especially the statement from the company that they wish him best in future endeavors generally means he is not leaving of his own accord or at best the decision was mutual. There are definitely differences in being an inwardly facing VP, Engineering and an Pre-IPO member of the executive committee. Maybe just wasn't his bag.
What would you expect the wording to be if it was his own decision?