If you have nothing selected, it will work because SONAR interprets 'nothing' as 'everything' when you ask it to bounce. If you have both MIDI and audio (soft synth) tracks selected, it will also work because you've specified a subset of the project that includes audio sources. But if you have only MIDI selected, SONAR will complain that there are no audio sources selected to bounce. That was probably the case when you were in the PRV, and when you closed and re-opened the project, it opened with nothing selected so the bounce worked.