The spike was in March which was the start of the COVID-19 pandemic in the United States/shelter-in-place, which was a time of odd behavior and so a spike wouldn't be too weird.

It turns out that dang posted a special whoishiring soon after which was massively popular: https://news.ycombinator.com/item?id=22665398

I thought I filtered out nonstandard threads in the query:

   WITH whoishiring_threads AS (
      SELECT id FROM `bigquery-public-data.hacker_news.full`
      WHERE `by` = "whoishiring" 
      AND REGEXP_CONTAINS(title, "Ask HN: Who is hiring?")
    )

Data science is fun like that, and surprisingly not the first time I've made that particular query mistake.