What is going on when our dog gets agitated at bedtime when we are going to bed and pet him goodnight?

he growls and snarls and looks around like he is being threatened. he sleeps at the foot of our bed. We have another dog that sleeps on her bed on the floor beside our bed. he growls and snarls at her too. he does this to all of us. my husband puts him in a submissive position and he stops growling but you can tell he is still upset.