I think Henro is on the right track - it looks like insufficient stress relief during original manufacture. The bucket is mostly made of low-alloy steel (for strength with ductility). But the front edge pieces and teeth have higher carbon for wear resistance. This also makes them more brittle - so technique is more important when welding the bucket together.
My guess is that the teeth were welded on AFTER the edge was welded on. As the tooth welds cooled, the shrinkage left a lot of residual stress in the edge piece because it was being restrained by the connection along the back. The cracks start at the front and migrate toward the sides as they go back because of the resraint along the back.
The cracks could have been there since new, or could have formed from something as simple as water freezing in the bucket, where the residual stress helped it break. The opening up of the one side is due to normal use after the cracks formed.
If this guess is accurate, the problem could have been avoided by different welding sequence, or by preheating more of the bucket before welding the teeth on.
