You'd know your cofounder best; I can merely give things from a techie's point of view.
The "getting something more than without you" thing is part of it, though that's easy to justify to any technical person who has tried to go into business on his or her own and failed. Most of us don't find selling natural :)
Unfair equity splits given the evolution of the role can be part of it - I've left companies before due to treatment I perceived as unfair or duties that were out of line with assumptions going into negotiations.
It could also be sheer boredom or overwork for a prolonged period of time. Startups are a grind sometimes, that's inevitable.
One thing which I think a lot of nontechnical founders miss is communication style. How are you communicating with the tech co-founder? Most people don't like to be disturbed when in the middle of coding; it breaks flow. Any communication runs this risk, but there are certain times when you should know better - do you hand out an assignment and then start calling / texting / otherwise interrupting during a time that you've just ensured the tech co-founder will be working trying to make that idea happen?
It could also be that a better opportunity has simply come up. What makes it "better" may not only be the money, and will vary from person to person.
Really, the best thing you can do is have a chat with the co-founder and get some honest feedback on the reasons for leaving. There may be hard truths, but it's better to unearth those and deal with them early.
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