The crates can be found laying next to the Verdant Meadows air control tower. If the player runs towards them, the crates will knock the player down, slightly damaging Carl. If the player runs on them constantly, they will eventually cause Carl to die.
The crates are in fact programmed as a movable debris. The same event happens with trash bags, tires, and windshields (if the player constantly stomps on the hoods of vehicles, it will eventually fly off) thus making the behavior of the crates game mechanic induced.