It's happening again!

Another plant dead in the rockery. Same suspicious set of circumstances. This time it was my Silene Maritima. Planted last year flourished, flowered and then died off over winter. New growth was coming on strong then pow! I found it dried and shrivelled. On inspection the base of the stem just under the soil was broken and ragged and the roots seemed swollen with no root hairs to speak of. same thin happened to my convulvus Cneorum in the same spot (hence the silene) and to both Dianthus deltoides nearby. All of which only lasted a year. Other plants such as the aubretia and saxifrage are unaffected. I can't see any obvious bugs. any ideas? Fortunately the silene seeded and there are now quite abundant groups of seedlings around where it was.