Technically it's not Massdrop made, it's a collaboration between the OEM and Massdrop. So while the reason behind the product and the tweaks and enhancement to the product are Massdrop driven, Massdrop isn't an actual OEM. They retain exclusivity and have products made with their brand and licensing, but the 'made' is a bit of a misnomer. They don't actually have production facilities themselves, they are simply the driving force coordinating design+production+distribution.
They'll probably break up shipment of the 21k units between now and then, but shipping internationally is not exactly as easy as going down to your local UPS store with 10,000 boxes of shit (which would be TERRIBLY cost inefficient). Depending on how many units fit in one container, they may send some to the distribution warehouse earlier than others, but the goal is to have them all shipped by December 18th (if I read this correctly). That would mean that between now and then Sennheiser is responsible for receiving the parts for probably 24k units or more I don't know what their buffer is, but it's to account for returned/damaged/non-functioning units received by the end user). They'll get the parts in Ireland, which will almost certainly be from international origination, which is ANOTHER headache, and then start the manufacture of final product. Then there's QA testing, packaging, bulk packaging and shipping.
I've been out of the game for a while so I don't know how best to coordinate that aspect. I don't know the size of the box, so lets say 12x12x5. Approximately 4600 boxes that size can fit in a standard 40' shipping container, so is it worth it to go through the process of loading and shipping one container every 4600 units? Probably not, because you have to figure in customs fees (and paperwork... so much paperwork)
It's likely that they'll wait to ship at least the first 10k at once, if they do break up the full 21k units. So you have to account for build time, QA time, packaging time, bulk packaging time, load time, ship time (this is going on a big ass boat across a big ass body of water, after all) receiving time, inspection time, probably a secondary QA spot check time, then final distribution. Final distro is going to entail a shitload MORE work because they have specific serial groups assigned to specific purchaser groups, so they can't exactly shotgun these things out the door. Plus there's warehouse space they have to account for in costs, as well as headcount; the fact that we're paying $200 a pop is cool and all, but every day that passes eats away at their bottom line.
So you're looking at a supply chain that starts somewhere overseas, hits it's manufacture point in Ireland, ships to New Jersey and gets sent out from there. It's not nearly as easy as shipping each one out as it gets made. The logistics behind an undertaking this size are freaking mind-blowing, and it's ALL done behind the scenes. If you have good logistics people, which MD seems to have employed, we will never know if there is any hiccup. Sennheiser has agreed to ship xx,xxx units by xx/xx/2017, but that's a part of their agreement with MD. MD has agreed with you, the consumer, to ship you x unit by 12/18/2017. That's their service level agreement with you (SLA) and it is a huge issue when SLA is breached. For something of this magnitude, I bet you dollars to donuts that all parties involved are going to light fires under SO MANY ASSES to make sure they stay within SLA.