It could fit their timetable to announce the Mac Pro update in October along with other Mac focused refreshes - OS X El Capitan, iMacs big & small, the Apple Retina 5K Display, etc - and then actually make the Mac Pro available to order midnight December 19, 2015.
This makes no rational sense in the context of Apple's normal practices for several reasons.
1. Apple doesn't try to hit exact, fixed in stone dates. The whole entire purpose Apple having their own scheduled dog and pony shows is so that they can pick what date is right for them and
not have the date forced upon them. For the last couple of years, iPhone release around September? Yes. Some exact date in September every year? No. WWDC on same date every year? No. The iPads on same date? No. Mac Laptops ? No.
The only thing special about Dec 19 was that it was technically about as late as you could go and still arrive inside the "Fall 2013" window. Going OCD on that date would be bizarre even for Cupertino kool-aid drinkers.
2. Try to mimic the 2013 announcement schedule is even more dubious. Effectively it has fundamentally different agenda. It went like this
June 2013 "We are going to stop selling the current form factor.... if want this one better buy relatively soon" ( i.e., sneak preview of new Mac Pro )
October 2013 "We told you should buy the old form factor because we are stopping selling it now. " (i.e., "soft announce" the new Mac Pro ... still no firm date for shipping, just Fall 2013, but more details now. )
Dec 2013 Fall technically ends in the month so ship something...
There is no likely radical form factor change coming. Any tweak they may make (e.g, add an inch or so in height and width ) isn't going to motivate many to get the "absolutely smallest" container before they disappear. Any physical tweak would probably be to enable a better thermal/power envelope; that's actually what more than a few folks want.
Announce and ship 2-6 weeks later is likely inside the same quarter. Announce and ship 12-16 weeks later is likely to suppress sales for whole quarter. Remember the iMac screw up (2012?) when they couldn't hit dates but had stopped shipping the current iMacs so it drove overall Mac sales down for the Quarter. I'm sure someone at Apple does. If can't get high volume shipments of Xeon E5 and TB v3 out of Intel unitil 2016 there is zero good reason for Apple to announce early and suppress whatever is left of the Mac Pro 2013 sales.
As one of those watching when it went live, and seeing the orders almost immediately balloon out to February and beyond, it would fit Apple's make-em-wait-dammit schedule (or, you'll-get-your-Mac-when-we-get-our-components).
There is leveraging a slightly scarcity effect and then there is being rude. One contributing reason demand ballooned is because they 'announced' so far ahead. Apple's standard practice is to stop shipping he current after the announcement. In 2013, compounded on top of that because has pragmatically turned off the EU switch back in February, 11 months previous.
So a 2015-2016 strategy of 'starve' folks who need a Mac Pro to get some work down for Nov-Jan is extremely likely going to piss off a decent number of folks. It is a tool for jobs that aren't on Apple's dog and pony show schedule. All that primarily does is make the HP, Dell, Lenovo workstation sales force job easy. "See Apple? One day when you need another workstation they will yank the product off the market and you'll be hung to dry for a couple of months."
And I of course would curse myself for not getting my order in the minute after midnight (of all things, my credit limit didn't go that high)
If Apple simply waits a bit to let the factor to get a couple more weeks of production built up then demand/supply mismatch won't be so high and so long. Too much scarcity invites hoarding and speculators to jump and makes the problem worse. The Mac Pro doesn't need that. Intel will start to ship to System vendors before their announcement date. Apple can use some of that to build up some inventory a bit higher than what they had last time. There still will be wait times but they don't have to be measured in multiple months.
[ Aside: I think the new MacBook is probably an exceptional case. I suspect it has been on a very tight leash because they are going to kill this v1 instance off relatively quickly. So they don't want any inventory in the channel when they do. Skylake Core M and TB v3 is a far better fit for a one port wonder than what is out there now. ]