Do I need to reason why I use a particular technology in my research proposal?
You have two (related) questions:


What does this mean?


This means you should explain why you suggest using YCSB (You already know the reasoning, given your last sentence.)


Does that mean I need to reason all the tools or technologies I have chosen to assist the research process?


No. If that were the case, the examiner would have asked you to. In general, researchers use direct/concise language. Do not assume that they actually/secretly want you to answer another question than the one they have asked you.