What happened on Stack Overflow on 31 May 2019?

I was analyzing the proportion of questions which received at least one answer within 48 hours, using Bigquery:

select da, avg(answered)from(SELECT a.id, date(a.creation_date) as da, max(case when TIMESTAMP_DIFF(b.creation_date, a.creation_date, hour) <= 48 then 1 else 0 end) as answeredFROM `bigquery-public-data.stackoverflow.posts_questions` a left join`bigquery-public-data.stackoverflow.posts_answers` bon a.id = b.parent_idgroup by 1, 2)group by 1

Here's what I get:

This figure shows the proportion of questions asked on a given date which received at least one answer within 48 hours, and while there is clearly a decreasing trend, something happened on 31st May 2019, because of which there is a discrete drop (from 68 per cent on 30 May to 56 per cent on 31 May). The values remain low after that point. What happened on that day?