He was actually trying to trade down, thinking he would still have his pick but decided against it. Concerned that he would get scooped. I can’t recall the article but it had a couple quotes in it from him.
*edit - it was sportsnet. Copied below.
THE RANGERS SPOOKED VANCOUVER FROM TRADING DOWN TOO FAR
It seems as though Vancouver assumed Nico Hischier, Nolan Patrick, Miro Heiskanen and Cale Makar would be picked in some order with the first four selections. At that point, Swedish centre Elias Pettersson was Vancouver’s guy and Benning believed he could trade down a couple slots, add an extra pick, and still get him.
Initially, Benning was talking trade with new Buffalo Sabres GM Jason Botterill about moving to the eighth pick. However, after the New York Rangers acquired the seventh-overall pick in a trade that sent Antti Raanta and Derek Stepan to Arizona, Benning shied away from the Buffalo deal, thinking there was a shot New York would take Pettersson.
“Now that the Rangers popped in there, I don’t know if… I’m scared to go all the way down to eighth,” Benning told Botterill.
“The Rangers were the wild card,” Bening told his staff after selecting Pettersson. “Like I had no idea, and they might have taken Pettersson if he was there.”