Three possible causes: a bacterial canker, the soil is waterlogged or the organic mulch is causing the trunk to rot.
Cankers should be easily visible on the limbs or on the trunk.
Elderberry doesn't like waterlogged soils: in fact it grows better when the soil is on the dry side with a few deep waterings. Waterlogged soil can not merely cause nutritional deficiencies and anoxia (lack of oxygen in the ground) but is the perfect breeding ground for all sorts of fungal diseases.
Finally mulch. It's self-explanatory: if moist organic mulch (especially acidic) is in contact with the trunk it will cause the bark to rot, leading to all sorts of troubles, including dieback.
