OP here. I don't know if anyone reads old HN threads but thought I'd clarify some things:
1. My scraper runs every 5 minutes, with a randomly-generated user-agent and never sends cookie headers
2. The charts are bucketed by half-hour periods, so even if the headline flips back and forth many times in half an hour, the colors are grouped together
3. Agreed that in the SpaceX situation (and maybe the Cuomo situation) the headlines change because the stories change. But, e.g., in the Meghan Markle situation, the first headline appears _after_ the interview is over. But that's something to watch out for!
And charts like this one[0] look (to me) like a clear example of A/B testing. But would be interested to hear other explanations!
[0] https://nyt.tjcx.me/articles/1163e0c4-e609-5cfa-aff1-b0945f1...