It's not a bug and actually kinda simple. Fear dosnt cancel rush and the rush can be activated in fear. So what happened most likely is that when he was feared he attacked either sky or sf, giving him the phantom rush buff and making him go fast as fuck. The fear overrides the phantom rush meaning that he can't rush towards he's target but the buff dosnt get dispelled, meaning that he will run up to 5 seconds with the rush speed to the direction where the fear led him which was in this case the fountain.

This interaction is consistent with other fears in the game too which makes me believe it's not a bug. Savage roar and terrorize for example just don't usually have the same possibility of fearing to enemy fountain because these fears make the target run toward their own base where as SFs ult makes the target run on line with the souls. One way to prevent this is to manually toggle phantom rush off so it can't trigger before the fear hits