Ted Sawyer, of Bullseye's Research and Education department, provided some advice. He wasn't able to see the cracks very well in the image, but it looks like the piece thermal-shocked on the way up in the fusing cycle and then healed on the way down, as you already observed. If you were to do this project again, he suggests slowing down the initial heating rate for that final tack fuse. Instead of 280 degrees F per hour, we recommend no faster than 100 degrees F per hour, up to 1000 degrees F.
Why must most tack-fused pieces be heated more slowly than fully fused pieces? Because the arrangement of glasses is characterized by thick and thin areas, and these are difficult to keep at a uniform temperature during the heating up phase. The thicker areas stay cooler than the thinner areas, resulting in thermal shock. (Slower initial heating rates are also necessary when doing a full firing of a piece that is assembled with areas of uneven thickness.)
We don’t have any really good ideas for mending this piece, since the only way to do so would be to fully fuse it together – which will destroy the relief quality of the tree. If you were to infill the crack with frit and tack fuse it, it would have a different texture and to some extent color than the rest of the background. If you were willing to modify the design, then you could cover the cracked areas with stringer or other design elements.
If those options don't seem like acceptable solutions, only way that we can think to make the piece seamlessly look as you originally intended is to remake it entirely.
For more information on how Bullseye glass behaves, see our "TechNotes 4: Heat and Glass" at http://www.bullseyeglass.com/education/#techbook
I hope this helps explain what happened. Let us know what you plan to do next!