Trainer Nate Campbell said he was "embarrassed" by his fighter's move.
"This man disrespected himself, his wife, the fans and me," Campbell posted on social media.
Journalist Jordan Hardy - who works for Premier Boxing Champions, which was broadcasting the fight - said Harper told her the walkout was because of money.
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"He walked out of the ring because he's not getting paid enough to fight and that he wants respect," she wrote on social media.
Harper had reached the bottom of the steps at the side of the ring with just five seconds gone in the round.
Fans began booing as he made his way along the walkway out of the ring and officials disqualified him at the one-second mark of the first round.
Britain's former unified heavyweight champion of the world Lennox Lewis shared footage of the incident on social media.
Writing on Twitter, Dave Coldwell, trainer of British heavyweight Tony Bellew, said: "Boxing...expect the unexpected!! Can't say I've ever seen this before!"
Harper now holds a record of six defeats and 13 wins, while Ajagba, 24, has six wins from six, including five knockouts.