Combination of rising nationalism and (via the Dominion system), sort of 'letting go' of daughter-colonies like South Africa, Canada, and Australia. Self-determination on the part of the above 3 was encouraged, but other colonies like Malaysia, Burma, and India had fairly lengthy struggles for independence and won out only when British power had generally collapsed, due to the inherent resources of the British Isles being not enough to compete with rising powers like the USSR, the US, and (violently) Germany.

In short, self-determination gave Canada, Australia, NZ and SA a great deal of independence even at the zenith of empire in the early 1900s, but when British industry and the economy in general was straining to compete with the US (and later USSR) in the forties and fifties, maintaining India in the face of stiff resistance seemed impossibly expensive. And without India (source of money and a captive market) and the Indian Army to act as a secondary garrison force in Malaysia, Indonesia and East Africa,
the rest of it slowly fell. Staunch American anticolonial stance in India didn't help, either.

My read from these sources at least-

1)'The Rise and Fall of the British Empire' -Lawrence James

2)'The Rise and Fall of the Great Powers' -Paul Kennedy