They're meant to be found as easter eggs, hence why LAGS calls them secret animations. For some, you get some hints in-game, but for other ones you get no information. LAGS posts youtube videos
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showing how to get them
after the updates have been out for a while though, as to not spoil the fun for everyone. He also posts little teasers for the next updates. If you want to get everything as fast as possible I can suggest you to go to the comments at the devlog: either the
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or the page for the current release
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, where people usually get answers faster than in this forum.
Edit: At this point I think all of the animations have been revealed at the devlog, but one of Annies's is bugged right now, so expect a bugfix soon.