Don was sitting in the audience throughout the first half, went up to receive his award and then went straight to the performance. The guitar was - I am reliably informed - electronically tuned back stage and placed on the guitar stand by one of the backstage people. presumably, the string slipped, or was knocked out of tune. Perhaps Don is used to having his guitar tuned and relies on it being in tune. Though it is surprising he didn't at least check the tuning before he started. Or perhaps the foldback wasn't clear. It would take a lot, I think, to stop, retune and start again with a live TV and radio and theatre audience.
all credit to him, though, for very quickly retuning in between the two songs.
mind you, I was hoping for American pie and Vincent .....
Derek