I heard a rumor / theorem that Apple planned to announce iMac / Mac Pro upgrades during the last event, but pulled them last minute based on the strength of the Surface Studio launch the day before, as in "oh crap, we better come out stronger than this".

Which would make sense if you considered how 'off' the presentation was: a looong time for not very much announced, and 25 minutes of saying not very much before we even got to that one real announcement.