Some of these are more hypothetical rather than me needing answers ????
1. Have you seen anything similar in previous years?
2. With the holiday season in the US that may cause a spike in online traffic, but potentially not globally. Did you run any special holiday campaigns targeting the US market?
3. Any changes to the paid budget or strategy that skewed toward US audiences in December?
4. Factors like hreflang tags and regional site performance might have slipped. Might be worth running a technical audit to see if the site is serving localized content correctly.
5. Unlikely given mass market impact, anything potentially happen on another domain leading to SERP changes in the other markets?
6. Anything to do with localized content if you do content outside of english?
7. Depending on where you otherwise see traffic, e.g. Europe, anything change wrt how medical information is presented. Could be compliance related?
8. Any particular sections of the site that saw the biggest decrease? Wonder if you got hit in some way due to the google update in December.