When the Doctor throws his sonic screwdriver, it lands on the ground with no hand mines around. When the boy is picking it up, the screwdriver is right at the base of a hand mine.
The doctor realizes he needs to go back and save the boy because Dalek-Clara can express "Mercy," something he says they shouldn't be able to do. In season 5 (The Big Bang), a Dalek begs River for mercy repeatedly, using that exact word.