Market risks have been accumulating, and today's sharp downturn suggests that traders are finally pricing in some of those risks. For instance:
1) Trump raised the possibility of a military coup in meetings with advisors. If he attempts it, it could be economically disruptive.
2) Valuations are high, at least by most traditional measures. (Some, including CAPE creator Robert Shiller, say that historically low interest rates change the calculation.)
3) The virus has mutated in Britain to become more contagious, and the UK is being locked down.
4) A Brexit deal has failed to materialize, and no-deal Brexit draws closer.
5) Lack of financial support for state and local governments in the stimulus deal makes state and local tax hikes likely in 2021.
6) Eviction moratorium was extended for only one month and expires at end of January.
7) Unemployment extension in the deal was shorter than expected and expires in mid-March.
8) NIH is investigating rare allergic reactions to Pfizer vaccine.
Of these, I think numbers 3, 6, and 7 explain today's downturn.