Ricciardo shocked the F1 world when he announced he was joining Renault at the end of the season.
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The Australian was tipped to move to Mercedes or Ferrari earlier in the year but he has instead opted to drive alongside Nico Hulkenberg.
Horner was disappointed with Ricciardo’s decision and initially thought the phone call was a joke.
But once realisation set in, the Austrian team’s principal explained the 29-year-old did not want to live in the shadow of Verstappen, who is expected to be the next big star.
“Daniel said he decided after a long flight to America that he wanted a change,” said Horner on F1’s official podcast ‘Beyond the Grid’.
“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.
“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 also explained the risk Ricciardo is taking by joining a team currently fighting in midfield.
“I thought he was winding me up to be honest when he rang me to say ‘I’m going to Renault’,” Horner added.
“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.”