I'm just an engineer and I don't do much on the business side, so take this with a grain of salt, but I think the fact that you want to start another company so

I'm just an engineer and I don't do much on the business side, so take this with a grain of salt, but I think the fact that you want to start another company soon is a perfectly good/normal reason on its own. You've found an opportunity you're excited to pursue, and you have full faith in the current leadership's ability to run the ship, etc etc. Wouldn't be anything I haven't seen plenty of times before.