The only way I can think of doing this is to beat up the metropoles enough so that they end up losing them anyway, not unlike what happened post-WWII. A worse WWI was mentioned, which strikes me as the most likely way. I'm not sure a power outside Europe can wrest them away; the USA managed to put a lot of its liberalistic ideas upon postwar Europe regarding imperialism primarily because Europe ground itself up into dust to begin with. Having the WWI analogue happen earlier can work as well, of course, if we want to go still earlier than that, but I'm not sure too much earlier than the late 1800's given the requirements of industrialization for such nightmarish warfare as caused by the world wars.
How strong were intellectual movements wanting independence? India had one, of course, so maybe strengthening them can do something ATL. You can even use the Wilsonian precedent of national sovereignty for the colonies a generation early if we use the worse WWI scenario, but that would require a still earlier POD removing a lot of racism and "the white man's burden."
Speaking of which, active independence wars are always possible. but I have a hard time seeing it happen successfully. Even European-descended populations in the Americas and Africa still had to put up with the mother country in their independence movements to begin with. No matter what happens in any scenario, the metropoles would not want to lose them, much more so arguably than post-WWII OTL, which was already an awful lot. I expect a lot more Algeria and Vietnam scenarios earlier... which will still make things significantly worse for all parties involved. Decolonization was messy to a large extent even OTL and if the colonies to break free I expect neocolonialism to pop up worse than what we have already.