1. Bounties & Bounty Hunters: During the Bull run bounty hunting was at its peak and all the bounty hunters participated heavily in twitter campaigns. Most of the tweets had some basic condition to add a few hashtags related to cryptocurrency I think that was the reason why the graph looks so tall during the bull run.
2. Trading Mentors & Media: A lot of hype was also created by all types of media around the world when they continuously posted around regarding bitcoin and how it went to moon. Moreover even every damn trader became a trade mentor because every coin was going up people gave calls blindly and became trading mentor. These mentors were giving calls on daily basis with these usual hashtags relating to bitcoin.
These can be the two reasons why chart shows such a long tower in December 2017.
The bounties immediately came to my mind, as the tweet history of bitcoin in 2017 increased on twitter. It is because of the number of twitter campaigns that the requirements are to tweet with the project name on the hashtag and bitcoin. This could also be due to the recent news that bitcoin's price hit nearly $ 20,000 and many have commented on it and tweeted their feelings in those days.