Possible Clues:

• Wake up at the back?
• Fielders run amongst players to take advantage of other movement
• Air driven back by a propeller
• Closely follow mistakes made by crew, crossing river
• Propeller-created wind
• Air behind moving vehicle
• Racecar-generated air current
• Line of least resistance past milers, perhaps
• Air behind any quickly moving object
• Closely follow error by group of students