Ricciardo had suggested during the post-Hungarian Grand Prix test that a fresh Red Bull contract would be sorted after his summer holiday in the United States.
He then informed the team two days later than he had signed a deal with Renault.
“Daniel said he decided after a long flight to America that he wanted a change,” said Horner on F1’s official podcast. “If you look at the rational reasons for that, it’s difficult to understand.
“Daniel had his reasons. I think it comes back to wanting to take on, his words, “a new challenge”.
“But I also feel he sees Max growing and growing in terms of speed and strength and he doesn’t want to play a support role I guess, for want of better words.
“It’s not that they were treated in any way different. They would have absolute equal status, as they’ve always had. I can’t help but feel that was perhaps a large part of Daniel’s decision.”
Horner said could understand Ricciardo’s decision to leave “if it was to Ferrari or Mercedes” but said committing to Renault, which is fighting at the front of the F1 midfield as it rebuilds its works team, is “an enormous risk at this stage in his career”.
However, Horner said he respected Ricciardo’s wish to “have a leading role, perhaps in a smaller environment”.
He added: “The competition between him and Max is intense. I think Daniel’s just decided the timing is right for him to check out and try something else.”
Horner said the reason he believes Verstappen was a key factor in Ricciardo’s decision is because Red Bull had met every demand.
That included a late request for a one-year deal to give Ricciardo flexibility if Mercedes or Ferrari was interested for 2020, or if Red Bull’s switch to Honda engines for next year proved problematic.
“We’ve bent over backwards to make it happen,” said Horner, who described it as “a bit like trying to convince a girl to go out with you that is being pretty reticent”.
“We gave Daniel everything he wanted and asked for, and it still wasn’t enough in his mind to say he wants to stay at Red Bull. So, it wasn’t about money, it wasn’t about status, or commitment, or duration.”
Horner learned the news on Thursday last week, the day before it was officially communicated, and said he was so surprised he thought it was a joke.
“I thought he was winding me up to be honest when he rang me to say ‘I’m going to Renault’,” Horner admitted. “I said, ‘is this a wind-up for the summer holidays’?
“It then became very clear that was his choice and you have to respect that.
“Renault is a growing team, they are committing resource there. Maybe it’s an inspired choice.”