I think Reino explanation is correct but not worded so that you understood?
The program only sent CQ because you had logged the QSO and cleared the fields. If you had waited and not logged the QSO until after you saw his 73 message it would have behaved as you expected.
Perhaps there is a difference in behaviour in how it works with RRR or RR73 endings but so few now use RRR sequence that I can't be sure!
