Two possibilities -- (a) they are changing/upgrading the Forum and needed to take it down briefly to accomplish this, or (b) they wanted to avoid the many complaints (visible on TMC forums) over the loss in value of recently delivered cars that are "suddenly" not state-of-the-art due to the dual motor and autopilot announcements. I say "suddenly" in quotes because both of these features were widely discussed on TM and TMC forums for the past year. A third possibility is that a software glitch just happened to develop on the same day as the "D" announcement, but I discount that.