I'm skeptical it has anything to do with your starter. I, and others I know, have pitched hundreds of (propagation) starters, both decanted and not, and haven't had this flavor result.
My money is on the hops. Being plants, hops are full of chlorophylls and other not-so-great tasting chemicals, and the levels of these chemicals will vary from year to year, grower to grower, etc, just like resins and oils will. These chemicals will leach into the beer, just like the good stuff will. If they're at high levels in the hops to begin with, the grassiness could overwhelm the good stuff.
I've gotten grassy beers before, but it has been conclusively due to dry hopping. I'm guessing you didn't dry hop your grassy beers...did you do a late addition with them?