Pretty much this.
The guy is campaigning on 2 things 1) his career as a businessman and 2) tax policy
Both of these things, I think, require his tax returns be released so the American people can really get to know what's going on with him.
1) It shows what he really gained in his business and how he functioned as a businessman. As someone else put it, tax returns are like the rosetta stone for a businessman. If he wants to sell America that this is why he should be our president, then he needs to be transparent with his returns.
2) He's running on the theory that richer people are being taxed too much, and that this should be lowered. If he truly believes he's correct, then he should show us how his life reflects it. Why is he afraid of this? Again, if he truly believes as he does about the tax code, then he should want to show all instances of it. Unless of course his tax code doesn't fit his narrative, in which case I think the American people need to know.
Basically, the guy needs to put his proverbial money where his mouth is. Not only is this a precedent, I think it's crucial to the arguments he's trying to make.