https://comicbook.com/marvel/2018/12/02/avengers-4-infinity-war-thor-stormbreaker-thanos/
"I think that key moment, though, is that Thanos was caught off guard," added Anthony Russo. "He literally just didn't know the power of what was coming at him. I mean, maybe he could have used the Stones in a different way had he understood what that weapon was, but it came out of nowhere."