I've had this happen in a couple of my beers, once in a ordinary bitter with Kent Goldings and another time in a SMASH pale ale with Cascade. I always suspected the hops, but now Denny got me thinking, so I looked in BeerSmith to see what kind of yeast I used. In the SMASH I used 04, but in the better I used WY1318.
But my water is low in sulphates, and at the time of those batches I wasn't paying attention to water. So...two unrelated hops...two unrelated yeasts...could be low sulphates???