Learning something new, seeking for more solutions to even a one problem often pushes scientists to change topics. Concentrating on one single issue should not be that long-lasting as the issues of different fields often are on the way waiting to be solved. Even it is instinct to change as human beings can not focus on one for a lifetime.

The cynical, and all too often accurate, answer is: they change to follow the money! Most science requires substantial funds to pay for the equipment & personnel required for serious research, so funding institutions, like the NSF & NIH in the United States, plus private corporations and private foundations and other government agencies have great control over WHAT gets studied and HOW by controlling the flow of money, choosing between funding this & funding that.

Changing the research topic is necessary to clarify the terminology. It's important for the researcher to identify and explain the main definitions.