There are many terrible interviewers and companies out there who would treat asking for clarification as a weakness so I imagine many interviewees avoid asking for clarification to avoid appearing "weak".
As others have said make sure you start the interview clearly inviting questions, but also if the candidate still seems reticent (they might believe it's a trick offer) maybe engage in a dialog as they start addressing the question.
For example immediately ask an open ended question about some edge case "What do we think should happen if the tree only has a single node?" or something like that. By immediately beginning a dialogue you invite more discussion. But bear in mind many candidates just won't enter into a discussion, it's not how they work, so try not to hold it against them if they are otherwise perfectly suited.
