This was Chase, in WA. Part of the problem may be that I used an address on the account application that didn't match the one on the state company registration. Rather than let me update the application, they required that I update the state registration to match it ($50 fee btw). (To be clear, both addresses were current and valid.) That may have set some kind of risk flag that increased scrutiny later.

But they were specifically worried about the transfers being scams -- at least, that's what they said. They insisted on calling my other bank to verify that I was in fact the owner of the other business account. And I know there's been a big increase in things like fake real-estate scams, so paranoia is understandable.

However, the way bank fraud/risk departments work is generally completely opaque. I've previously had Bank of America refuse to open an account, with no reason given and no possible recourse. And I can't imagine a much more vanilla, boring, good-credit person than me, so I have no idea what set them off!