Why is that?
1) He is invested in getting respect or the outward appearance of respect. That may be cultural and/or from feeling undervalued in childhood. And this need can be aggravated by being undervalued currently (for instance at work).
2) Your new-found assertiveness feels like the threat he fears most - the beginning of the end of the relationship. If "mommy' can refuse to do things, "mommy" might refuse to stay with him altogether. Then how will "baby" survive?
Its important to understand that some abusers are motivated by (1) and (2) while others are mostly driven by either (1) or (2). Figuring out which will help you to know how to maximize safety for yourself and/or strategize how to leave him.