I know exactly what he was trying to do. He was trying to pass behind the truck. Because of his hoodie and phone, he didn't see the second trailer attached. But I might be wrong on that, because he actually bangs his outstretched phone-hand into the end of the truck itself, so...
No, I don't know what he was trying to do. All I know is that he's dumb as a post, and probably dead as one too.
I think he realized the second trailer so he tried to stop, but it was too late