I'm a Windows user but that sounds normal. Since you're overlapping two clips with audio, you'd normally want to blend the audio together smoothly during the video transition so that's what PD's doing for you automatically here.

If instead you want one audio to simply stop when the new clip begins, or if you don't want any audio transition(s) at all, unlink the audio and video tracks first and then the visual transition will only affect the video. However, you'll then need to manually adjust the second audio track to keep it in sync after the transition.

Note that if you have the transition style set to Cross (rather than Overlap), you won't have to do any audio syncing.