How did Joseph Balicki get injured in chapter 2 of The Silver Sword?

Joseph has successfully managed to escape from the prison camp and is now making his way down to the village of Zakyna. In order to avoid a drunken German soldier, he hides in a roofless cage full of cargo, which he soon discovers is being moved across the valley in an aerial lift.
The cage is full to the brim as it is, but there's room for one more item of cargo, it would seem. Someone leaps into the cage and lifts the tarpaulin from the stack of crates opposite from where Joseph's hiding. Just then, another large, heavy crate is dumped into the cage, which bangs against Joseph's foot, causing him immense pain. He feels like screaming, but of course he can't, as he'd then get caught. So he falls back, groaning as he bites his lip.