i think they just do that at a certain time of the day. i do not really have an idea or a scientific basis, however if it happens must be something that is normal to them to do at a time in any given day.
It is a myth that roosters only crow early in the mornings, they crow all the time except when they're sleeping, eating, or chasing hens. It's a territorial thing, a way of keeping other roosters at bay.
