I would imagine that it may have been done through necessity; too many bodies to handle (for example during epidemics or war). Maybe it was done for sacrificial reasons?
Cremation as a funerary practice is thousands of years old and has been/ is practiced by a wide variety of different cultures across the world. Someone here might be able to help you or point you in the right direction, but if you do not get an answer that you find satisfying, you should try posting this question in /r/AskAnthropology- they are likely better equipped to answer questions on the origins of certain funerary practices.