RE: query28 Feb 2018 16:09
Because (i forget the exact percentage) there have been ample data driven articles suggesting buy backs, more often than one would think, result in a reduced share price. So, it wouldn't be completely out of the question, that the other posters suggestion of shorts closing, has some merit. i.e the shorters made an assumption this would be one of the cases where the price would go down, which it has, and would then close out upon completion of the buyback.
Not suggesting that is what is going on, not taking any sides, but the premise is not completely illogical to be ruled out.