I think by the time these things tend to happen, projects have outgrown their singular founder and usually have a large developer community (because issues usually take a long time to bubble up after an acquisition). So, it's hard to find a specific case of the founder driving the fork.
Monty forking MySQL after the Oracle acquisition is the only real example that comes to mind. Oracle being Oracle resulted in OpenOffice getting forked, too, but that was mostly driven by Canonical.