This is what they alleged:

Twitter claims in the cease-and-desist that Meta has poached dozens of former employees in the past year, some of whom “had and continue to have access to Twitter’s trade secrets and other highly confidential information” and “many” of whom have “improperly” kept Twitter documents or electronic devices.

“With that knowledge, Meta deliberately assigned these employees to develop, in a matter of months, Meta’s copycat “Threads” app with the specific intent that they use Twitter’s trade secrets and other intellectual property in order to accelerate the development of Meta’s competing app, in violation of both state and federal law as well as those employees’ ongoing obligations to Twitter,” the letter reads.