While I am not saying this is the reason, quite a lot of Twitter users jumped ship, and a fair number came to Tumblr, when Twitter’s new owner took over. They thought Twitter might change and become less welcoming towards them, while Tumblr has always been welcoming to almost anyone who wants to join.
There used to be a Twitter account called The Strange Log which regularly posted things like this. Looks like it’s inactive now, though.
While I am not saying this is the reason, quite a lot of Twitter users jumped ship, and a fair number came to Tumblr, when Twitter’s new owner took over. They thought Twitter might change and become less welcoming towards them, while Tumblr has always been welcoming to almost anyone who wants to join.