Since he was Director of Marketing, and getting better at marketing to a new generation is one of their talking points, I suspect his departure may not have been on the most pleasant of terms.
I'd suggest that is entirely unwarrented speculation. Simon was one of the four-or-five ex-directors who took the risk of financing the management buyout about 10 years ago....having pulled-round the company and run it successfully since then, it seems evident from the public information readily available (via Companies House, etc.) that they simply thought it was time to realise their efforts and investments. (And none of us are getting younger, everyone hopes to retire at some stage!)
And, as I said above, a simple confidentiality clause is absolutely normal in such a situation. I left as a director of a company about 15 years ago, perfectly amicably (still have lunch with the present directors occasionally), but the agreement at the time prevented me from disclosing any information concerning the business, or setting up in competition to the new owners. Totally standard procedure.