Tim's departure announcement is timed between product announcements. Although, not a surprise to anyone

As for Ternus' timing:

A)Not very Apple to have the CEO do one last launch while on the way out the door (want the full throated, 1000% commitment in execs)

B)Gives John stage for first time as CEO at September Keynote (historically a BFD)

C)It felt right, among all the other time-slots and factors to consider

D)John gets to announce the next One More Thing, and own it. Would be odd for Tim to announce the One More Thing and then resign.