I have read that decolonisation really began after WWI. So why did Britain begin to lose colonies (such as Australia, Canada, Newfoundland, and New Zealand) that were granted Dominion status from 1867 through to 1907?
What you need to think about with regard to this question is that the kinds of colonies that were given Dominion status during this earlier period were very different than the kinds of colonies that became independent after the World Wars. The colonies that you mention here are all colonies populated almost exclusively by British people (and some other kinds of people of European descent). The colonies that gained independence later were colonies in which a relatively few British ruled over large native populations.
After the American colonies broke away from Britain in the late 1700s, the British realized they had made a mistake. They realized that it would have been better to compromise with the colonists and keep them within the empire while giving them much more autonomy. When the British were faced with other colonies wanting more autonomy, they granted it. This was done first in the British North America Act in 1867. The British granted this kind of status to these colonies because they could be expected to remain connected to the British Empire because they shared a common heritage and language.
The British gave these earlier colonies autonomy because they felt that the white colonies could have self-government and yet still remain loyal to Britain. They did not want to suffer a repeat of what happened with the colonies that became the United States.