hashtag planting

Piggybacking on a popular or trending hashtag to increase the chances of people seeing your tweet about an unrelated or related topic.

For example, if you are hosting a video game event and it's near the time of the popular "Comic Con" convention, you would do some "hashtag planting" in your event tweets and use the already popular #comiccon so that all of the people looking for info on Comic Con would also see your tweets about your event.