If buy into all of the "the sky is falling" buzz then perhaps just a temporary run of folks buying before the current design is discontinued. (at least from "new" as opposed to used/refurb market). After users buying hardly anything for more than several months no sane retailer was stocking a deep inventory.
The iMac dried up early but all indications are that they expected to resume shipments much sooner than they did. This Mac Pro "sneak peek" seems far more a warning period so folks can freak out for a couple of months before Apple stops selling the current design. It is only going to cause even more drama if Apple turns off the spigot long before they can ship something new. They certainly aren't making alot of new friends in the EU where supply has dried up months ago.
Sometime this year , sure. That has been known since June 2012 when Apple openly targeted something new for 2013.
The more pressing question is whether Apple has completely shutdown their contract manufacturing for the current Mac Pro already probably months before the new Mac Pro arrives. Appe actively tries to carry just 2-3 weeks of inventory in the channel. That super lean inventory approach is a double edge sword on product transitions. Get the timing wrong by several weeks and have screwed up substantially.
They had to have reasonably expected a small bubble here in demand when a subset of the population said " Heck no, I'm not going that direction". They pragmatically cancelled the dual package version in next generation. Minimally some of those folks may be looking for a "one last iteration systems". Some folks are gong to have "larger than cost of New Mac Pro" investments in PCI-e cards and attached infrastructure who have put off making a Mac 2012 purchase for last 6 months and staying on OS X is still the best fit for them.
I just bought the base Mac Pro on Wednesday through the Apple Education store. Even though the website says "Ships in 24 hours" my estimated ship date is 3-5 business days. Makes me wonder if they are now just doing "build to order" on these and have none in stock. I didn't change anything from the base model on my order, which is what they normally keep in stock.