Listen, you'd be surprised what an elf can consider to be 'serving his master's interest' - our elf at home used to take it into his head to toss our rooms for 'contraband' periodically, to curry favour with our mother. And if he found anything in Reg's room, you can bet he'd plant it in mine, because he decided it'd 'break Mistress' heart' to learn Reg had any secrets.

It's possible that, knowing Harry is not Voldemort's real son, this elf has decided that Harry is in no way its own master, and therefore it is not bound by the laws of its enslavement.

Another question is why now? What's set this elf off this year, when it had access to Harry all his life? I wonder if Harry left something where the elf saw it, to suggest that he's not entirely loyal to Voldemort? That might have convinced the creature to go after Harry.