We can't tell what caused it, though heavy branches that might have been strained by winds or an ice/snow load (not recently, of course) could have weakened the branch junctions or created fine cracks that only now failed and split. If there was interior wood decay that set in from a prior bark injury, there would probably be more hollowed-out and dark wood in those areas (which doesn't appear to be present, but it's hard to tell), so we think physical damage is more likely than infection as the original cause. Drastic freeze-thaw temperature swings can split tree and shrub wood, but here too, that doesn't seem likely of late, given our weather patterns this season so far.

That's an impressively large Daphne shrub -- they are so sensitive to root drainage that they often decline before they get that mature, at least in our growing conditions -- so it may just need to be cut back to rejuvenate growth in the areas that split. Granted, it looks like that will remove lots of growth, but there is no way to repair the split in a way that guarantees it will not worsen or cause branch dieback due to the damage to the vascular system. For now, you can wait and see how it resumes growth later in spring, and evaluate it for pruning needs at that point. Although you can manually wrap the split areas with grafting tape, stretchable plant tie, or some other sturdy support wrap, the wood won't fuse back together again. If the root system is still healthy, a shrub this well-established should be able to regrow well from pruning if you wind-up needing to remove many of those split branches.

Miri