There is a cap because it would be a huge request to the server to output all of the followers for certain streams. Keep in mind that some channels have more than 400 000 followers. As a workaround, you can set a few parameters to your request, such as ?direction=ASC&limit=100&offset=0 (Documentation) and get the first page, and then you change the offset to get the second page, and so on. I would suggest to check if a certain viewer is a follower only when it’s required.
Thanks,
Schmoopiie!