Close in meaning to an explanation, a justification is specifically for defending or explaining an event. The idea that he was just trying to scare his little sister was a pretty weak justification for the elaborate gag.
Like its close cousin "justice," justification is derived from the Latin justificare, which means "to make right." When you offer a justification, you're trying to make something right—or, perhaps, even just. The notion that animals don't have emotions is one of the key justifications for allowing them to suffer. For many people, though, the fact that they suffer is justification enough to prevent it.