This delay in both 10.5" and 12.9" supports the theory that I have re next iPad launches. Here's what I think Apple might be about to do...
Reposition existing 9.7" Pro as new entry-level replacement for Air. Benefit: production line up & running & same form factor as Air but truetone screen gives worthwhile upgrade especially with the speed bump there as well.
Introduce 10.5" Pro with edge-to-edge screen but probably still with top & bottom bezels albeit slightly smaller ones (as in Samsung phones). This should still give a 10.5" screen in roughly the same form factor as Air. Updated electronics as well. Benefits: Pro now has more differentiation from Air. More screen in same form factor plus 1 generation later electronics is pretty significant. Maybe more memory too.
Introduce next version of 12.9" Pro but, and here's what I think a lot of people might be missing, do it using the same edge-to-edge form factor as the 10.5" version. Benefits: The Pros have a common form factor so the lineup seems more coherent but crucially there has been anecdotal evidence, some of it on these forums, that some potential purchasers or even owners of the current 12.9" model love the big screen but find the form factor just a bit too big (e.g. like the iPhone vs iPhone Plus dilemma that some people have). Edge-to-edge allows Apple to keep that big screen option but slightly reduce the downside in terms of size and weight of the device.
If my theory is true then it's interesting to see that the two models that are rumoured to take longer to ship are the two edge-to-edge ones.
Finally, I note that by keeping at least the upper and lower bezels it doesn't steal the thunder of the probable full-front-screen iPhone 8 (or whatever it's going to be called) launched a few months later.
I really do think all this could hang together really well.