I think this guy is a deviant sexual predator.
If he had never met her before, then this is a random attack spurred by his sick impulses.
However, I believe he was waiting for her, likely having cases her out prior. As a construction worker, these early hours are “normal” for him. Had he been in the vicinity before to grab a coffee or some thing and saw her? Decided to watch her routine/schedule?
Also, and this is probably going to be an unpopular opinion, but she was a tall, larger girl. He might have some kind of fetish and chose her for that reason.