Why would a recruiter want to know which other companies I have interviews with?

I am a mechanical engineer, and have never worked with a technical recruiting firm before. I interviewed with them 2 weeks ago, and they liked me. The position they had for me fell through, so they said they would let me know when something else came through with my expertise. I spoke with the recruiter today, and he asked me if I had any other prospects. I told him I had an interview tomorrow and he asked what company it was and who I was meeting. Is this typical? I am concerned that he will sabotage my interview, cause he gets paid by placing me himself.