This is entirely from my impressions in conversation with Vanderwal and nothing he's specifically said, but as much as he and his family appreciated Marietta, the school and the town, and all the things they've done to welcome and appreciate him, he's still a Michigan kid at heart and if the right MIAA job came open, it always felt like he'd have a hard time saying no.
Now, couple that with the financial issues Marietta's been having and the fact that his kids are just getting to HS age - timing makes sense. Trine is a better gig is pretty much every conceivable way, closer to home, more room for advancement, if he's still thinking about coaching at a higher level someday.
Heck, he's already had some success recruiting kids from NW Ohio down to Marietta. Really, really tough to say no to that one - especially when it sounds like Trine went after him pretty hard.