I feel sorry for you all that have pre-ordered this lens. The news of this possible delay only gives me (and I'm sure many others) more time to save up for this lens, which in my opinion is overpriced, especially the UK market.
I'm pretty sure they found some issue with the lens after the announcement, would you prefer them send you a defective one?
I'm pretty sure they found out after the announcement, that they need finally close the design and think about the production. Oh wait a minute. They found some issue? So it was a not tested prototype? I think they counted on research data from Curiosity but this spiritless robot melted the wrong stone and NASA has to find another one - this is the real reason for delay. And as we all know data connection with Mars is so sloooow....
Your forgetting about the pre-production model accidently left at Tranquility Bay by Apollo 11 in 1969.
As soon as UPS or Fed-Ex deliver it production can start