I didn't notice this being mentioned in the webcast or in any of the follow-up articles I've read on the presentation: while the timelines for 6-months-to-orbit seem insane, I believe the underlying drive for an aggressive schedule here largely derives from Mars launch windows. [1]

We have a launch window to Mars open from July 2020 - September 2020, the tail end of which is almost exactly one year away. Starship will have to be orbit capable well before this window, such that the flight and maneuvering characteristics are understood well enough to reasonably justify taking a crack or two at an unmanned cargo Mars mission within the 2020 window. Musk might not have explicitly mentioned it in the presentation, but I would bet that the Starship teams are internally being driven to these targets.

FTA, Musk is quoted in 2017, saying SpaceX was targeting uncrewed Mars missions in 2022, but given his ambitions and depending on the progress of the Starship/Super Heavy, I can see them actually aiming for 2020 such that any failures would still guarantee a path to making the 2022 window.

All of this is my own amateur speculation, would love to hear corrections or other ideas.

[1] https://en.wikipedia.org/wiki/Exploration_of_Mars#Launch_win...