Looks to me like 2020/2021 has some wilder spikes that other years. I wonder if that's because people are much more focused on Twitter during the pandemic. The site certainly makes the world feel like an awful place with that stupid trending panel forcing it upon your eyes.
The only thing that can realistically cause a significant spike in tweet activity is a major news event, and major news events are typically unlikely to be neutral.
Critically, the baseline is fairly static, so the extremes in this graph can only be reached by fairly major events that are capable of drowning out the typical everyday tweets.
