I am not so surprised about this. Many of bitcoin users are young people and they use tweeter. Even bounty campaign frequently spread the word about cryptocurrency there. In my point of view, maybe this trending happen because people are getting enthusiast with current market situation. Some people panic because of the recent bitcoin price fall and some people are optimistic. It also can be caused by bots. I don't know the real reason but I think it's still good to spread the word about bitcoin as the number of tweeter user is very big.