Bristleworms do not stay in a specific "burrow", so IMO
it is unlikely that the shells are anything but a coincidence with the bristleworm sightings. In general, bristleworms lack any appendage to catch a moving invert, from my experience they can only grasp after some effort with their mouths (used to watch mine carry away algae chips). They scavenge dead flesh, otherwise they would likely be seen following fish and snails all day and night which I have never seen.
I'd suspect water conditions, holding conditions at the LFS
(which makes them die prematurely), or a different predator...I have had the same 6 snails in my 55g for 2 years now and have tons of bristleworms.