The term at issue is not "master", which has several meanings - some noun, some adjective, some verb. It's "master bedroom", which has a historic meaning that these days very few people know or care about - but some do, and that's what is prompting the agents to take action in their best interests.
If the same £4M London property is listed with two agents, one who uses "Master Bedroom" and the other who uses "Principal Bedroom", and the eventual buyer is put off the first agent because of their language, that's a problem that first agent could easily do without.