Here's a personal impression of a busman's holiday:

A busman carries people around in a bus for a living. When he has a holiday, he finds himself back on the bus to travel to wherever he wants to go.

A busman's holiday, so restricted, still involves what he does to earn a living.

Broadly applied, when one goes on a busman's holiday, no matter the profession, it simply means that much of what you are doing for pleasure is tied up in those same activities you encounter in making a living.

The term busman's holiday may be used either positively or negatively.