In-laws

I've been with my significant other for 6 years now. I've never gotten into any fights with his mom and dad yet whenever they have an issue with him they attack me. As though I've convinced him to say certain things or to feel a certain way. They seem to be harboring bad feelings towards me that only come out when they are upset with him. Otherwise they claim to love me and are always saying how we are soul mates. Please any info is much needed. I just don't understand why they act so venomous at the strangest times. Do they actually not like me and are just faking it? That's why they are sooo mean to only me when mad at their son?