If does keep coming into esterous (unbred during the first cycle) then bucks are going to go to chasing again the following month and subsequent moths until does are no longer coming in esterous. A secondary rut is quite common in most places, but I do think we have multiple cycles of rut due to a lot of varying reasons. I some areas the buck doe ratio is skewed highly toward the number of does. Bucks just cant breed them all in their first esterous cycle. In other areas there is probably a herd age structure that has too many juvenile bucks and not enough older mature bucks. Then there is a small percentage of the doe yearlings that come into their first esterous late as well. I have personally witnessed the chasing of does, fresh scrapes and breeding behavior in April.