To me this sounds like verticillium wilt. This is a bad one! And unfortunately elder's are suceptible to it. It effects the vascular system of the tree and prevents it from taking up nutrients. It is soil-borne and lives in the soil for years, but there are quite a few plants that are resistant to it/somewhat immune. Some plants may have it and not show symptoms.

As far as what to do about it - if the shrub essentially "dies" every year and looks poor, you may just want to remove it. If pruning out all the dead stuff, watering well and fertilizing can revive it, you could do that. If it is VW, it is already in the soil, so removing the tree immediately won't save it from "spreading" or anything. Do destroy any wood you prune out of it though(burn, etc.)

If you do remove it, and want to replant something there, you could fumigate the soil where the elder was. I'm not sure that you could really do a complete enough job to get rid of it though, so it is probably best to just plant a resistant shrub. I attached a link with more info about VW, it also has a list of resistant plants.

Anyway, don't panic..it may not be VW. It'll have to be identified first before you take action.

Here is a link that might be useful: Verticillium Wilt