without more info or photos.. so hard to tell. Also are they lilies that die after blooming? could it be a root rot? could be a kind of root borer? just a pathogen that didn't show its ugly head until environmental conditions were right...
Also extra fertilizer can hurt them. Pushing
plants to grow can in some cases, reduce their ability to fight on insects and pathogens.
is there spotting on the foliage? does it look like wet marks? is the yellowing inner vainel? did you dig a bulb up and smell it? does it stink? it is mushy?