When I use the copy followers feature by Crowdfire, I sometimes get an error "you hit Twitter limits, retry later", even without following anyone:
This happens especially with accounts which have tens of thousands of inactive followers, or when you have already followed their followers.
The reason is that Crowdfire is asking the list of followers which has rather strict rate limits. If the listed users are all discarded, it may take hundreds or even thousands of requests until you're shown an entire page of results from which to pick people.
One solution is to slow down the requests when you see that they keep spinning without producing results. To do so, in Chromium/Chrome, you can press F12, click "Network", click "No throttling" and select a slow connection from the dropdown.