He drifted that way because the ball is headed out of bounds. Your picture shows that IMO.
Imagine that he is not leaning toward the sideline in the picture. Where does that place his head relative to the ball? It would hit him in the face. If he were another foot inside the line, he might have to reach up a bit to catch the ball. If he were two feet over, he could jump and still catch the ball. Professional WRs make that play every week. The throw was fine.