Bitter tasting yolks - any ideas?

I've kept a small flock of 4 hens over the past 8 years and just recently we have had a few eggs that have yolks that have a bitter after-taste, not so good in a dippy egg for breakfast 
I feed them Allen & Page layers pellets, mixed corn at night, apple cider vinegar in water occasionally.
I suspect they may have eaten something yucky from the garden - Any ideas??

Hi iv never herd of this before and had a look on the net, the only thing i could find is as joyfull said its down to somthing thechuck ate, have you any plants with smelly leaves