To justify something means to prove it is right. To explain something means to clarify or make understandable using reason or logic. Consider the following examples.
Can you justify why you spent $100 at the store?
Be sure to justify your decision in writing.
Can you explain why you spent $100 at the store?
Be sure to explain your decision in writing.
stated, asking someone to justify a task, action or behavior is more likely to produce a defensive explanation.