Helmut Marko, who is Red Bull motorsport advisor hasn't understand Daniel Ricciardo, who made a decision to join Renault.
"I don't understand it. It was a very strange situation anyhow.
"The negotiations were difficult but Wednesday before the Austrian Grand Prix we were talking for two hours and came to an agreement.
"During the weekend, we were talking and renegotiating again.
"In Hungary, he told Mr [Dietrich] Mateschitz and me that he was OK with everything and that he was going to sign during the [post-Hungarian GP] test on Tuesday. But he didn't.
"On Thursday, he then called and told me he was going to Renault. I can only assume that maybe he doesn't believe in the Honda project or that Renault offered him a lot of money."
Ricciardo has won two races with Red Bull this season while Renault is yet to score a podium as it continues to rebuild its works team.
Renault has also admitted to being disappointed it has not made more progress as a race team and as an engine manufacturer in 2018.
Marko said it is "going to be difficult" for Ricciardo to perform his customary 'shoey' victory celebration "in the near future".
He added: "Including his time in the junior programme he has been with Red Bull for 10 years.
"He said he needed a change of surroundings. I can understand that, but it's a pity because he is one of the strongest drivers.
"His overtakes are great. They come out of nowhere and sometimes his opponents don't even realise they are being overtaken," told Marko Austrian broadcaster Servus TV.