This technique is used purposefully to give you time and space to think about the question and provide an open ended answer, to provide detail, thoughts, feelings, experiences, etc.

"Maybe X happens...?"

Yeah... (Silence) It happens when . . . and then I feel . . . and I want to . . .

Just stating "Yeah" doesn't give much to process. They're waiting for you to fill the silence with what's going on, why did you answer yeah, what does yeah mean, how is it impacting you, etc.