What workflow ends up with a Customs and Border Patrol officer connecting that domain name to Stenberg's ESTA application?

Here's a guess: some blacklisted piece of software uses curl and credited "daniel@haxx.se" as required by its licence[1]. Daniel was then mistaken as a creator of that software, and his email address flagged for any ESTA applications that use it.

edit: apparently I'm not the first to think this: https://news.ycombinator.com/item?id=14643525

[1]https://curl.haxx.se/docs/copyright.html

so he is 4chan