Do you have close neighbors by any chance? Is it possible that you have a neighbor who doesn't like it when the rooster crows? Sounds like a possible poisoning to me.
Here is what a farmer once told me and it has worked time and again. Roosters, especially young ones about 1.5 years old, go through a phase of sorts where they try to test their dominance in the pecking order. YOU have to be the bigger rooster, otherwise he'll keep chasing you. So, spread...