Why Did NKOTB's Jonathan Knight Walk Off Stage Mid-Performance?

New Kids on the Block member Jonathan Knight apologized after walking off the stage mid-concert in New York on Thursday.

According to CNN, Knight was visibly out of it during the band's performance of their '80s hit "Hangin' Tough" and then walked off the stage when he was supposed to sing a solo. Although fans chanted for him to return, he never did.

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"I'm sorry......" Knight tweeted later that night.

Knight's bandmate Donnie Wahlberg also tweeted about the incident, but in a much more upbeat post. "To recap- Joe was especially dazzling! Jordan was especially amazing! Danny was especially solid & Jon was especially shy! #Allsystemsnormal" he said.

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Knight has a history of anxiety and panic attacks, which he revealed in a 2001 Oprah Winfrey interview. "The whole New Kids thing was crazy," he said at the time. "The more it grew, the more I just felt like I was trapped. I just always had this feeling inside me of always being nervous and afraid of situations."

NKOTB is slated to begin a tour with fellow '90s boy bands 98 Degrees and Boyz II Men this summer.