Currently, thresholds are put in place such that retweets more than 5 minutes after the last retweet will not be grouped together. This is done because the visualization becomes too messy to make sense of roughly around this point.
An alternative mechanism would be that retweets after this amount of time start forming a new grouping, rather than simply being dropped off. Figuring out how to link these events visually will be a challenge and needs further examination.