The airlines are just pinging the ESTA system.
If they bring someone to the US without ESTA permission they get fined $5000.
The same thing happened two weeks ago to my boss, he had an ESTA permission issued until Nov 2018 but it got revoked without notice or reason and he only found out at the airport. He still doesn't know, even though he has an Official Passport (sometimes called Service Passport) and was on diplomatic mission to the UN climate conference in NY.
Airlines don't have very good systems for handling visas and ESTAs. I personally never cannot checkin online for the flight from Europe to US since I'm on a work visa not an ESTA. At the counter they just check my visa, type it in and check me in.
Might be that in this case the airline person just typed the wrong ESTA information and either didn't find it or found one that was rejected, or he simply just didn't file an ESTA or it was expired.
The same thing happened two weeks ago to my boss, he had an ESTA permission issued until Nov 2018 but it got revoked without notice or reason and he only found out at the airport. He still doesn't know, even though he has an Official Passport (sometimes called Service Passport) and was on diplomatic mission to the UN climate conference in NY.