"Adele's Grammys performance: What went wrong? "- CNN.com

" more
detailed explanation to reporters after the show.
According to Variety, he said that as the piano was being
lifted onto the stage, a microphone in the piano
dropped, which caused the audio glitches heard during
the broadcast. Technicians tried to correct it by going to
a backup system, which caused the feed to be lost for a
second, he said...."

http://www.cnn.com/2016/02/16/entertainment/adele-grammys-performance-feat/?iid=ob_mobile_article_footer_expansion&iref=obinsite