Looks like it's the prankster's fault...

UPDATED STATEMENT: Customers removed for disruptive behavior
Upon landing, the crew was debriefed and multiple passenger statements collected. Based on the information collected to date, it appears the customers who were removed from DL1 from LHR to JFK sought to disrupt the cabin with provocative behavior, including shouting. This type of conduct is not welcome on any Delta flight. While one, according to media reports, is a known prankster who was video recorded and encouraged by his traveling companion, what is paramount to Delta is the safety and comfort of our passengers and employees. It is clear these individuals sought to violate that priority.

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