Per a report from Pro Wrestling Sheet, WWE sent the Young Bucks an actual cease and desist over their constant use of the "Too Sweet" hand gesture that was popularized over twenty years ago. The gesture rose to prominence due to the "Kliq" and NWO, comprised of Triple H, Shawn Michaels, Sean Waltman, Kevin Nash, and Scott Hall.
Pro Wrestling Sheet stated that WWE wants an agreement promising that they'll not infringe on WWE's intellectual property. WWE has asked the Bucks to stop using WWE's intellectual property (the "too sweet" gesture) at events and on merchandise they sell. Otherwise, they risk paying $150,000 in damages.
WWE registered to trademark the gesture in March 2015, but was abandoned in 2015 because WWE didn't respond to a USPTO inquiry within the six month allotted time period.
Young Bucks and other Bullet Club members have several shirts that include the gesture and the phrase "Too Sweet" on them.
Ryan Satin of Pro Wrestling Sheet says that the Young Bucks have consulted a lawyer regarding the matter.
It's worth noting that Fightful has been unable to independently confirm the news, and were told that Vince McMahon didn't sell the news of the "invasion" to anyone backstage.