Shortsellers could be buying the bonds to create a partially hedged position. Any number of managed bond mutual funds could have been buying ever since the dividend was cut, as the credit quality was almost certain to gradually improve (with the debt reduction). With the outlook for 2020, that bond strength looks to continue.
FWIW, I think shortsellers will help to keep the stock range bond this year. From a fundamental point of view, it looks like the bottom is in on the stock. However, absent a takeover, the guidance suggests that a major move up looks unwarranted at this time. Therefore, one might expect shortsellers to cover on declines and short again on advances.