As explained by most seasoned hunters, the number of animals to hunt goes down. This means the number of prey that the coyotes can actually depend on for survival has gone down. Winter is that season when animals like deer either migrate, hide, or their population completely goes down. For coyotes to get something to eat, therefore, they have to get together and cover a considerable size of the hunting field. The reason as to why they have to cover a larger field is due to the scarcity of prey, and thus have to come together and search for the scares animals.
During winter, as indicated above, there are fewer animals to hunt. Even those hunters who have the best holographic sight find it hard to locate an animal to kill. This makes the coyotes vulnerable, as they become the only animals available to be hunted. For this reason, coyotes would come together to provide security to each other. This is done by sounding alarmed anytime a coyote sees a hunter. This is in order to alert others of the impending danger.
It is true that coyotes are solitary animals, but this fact changes during the breeding season. During this period, you will see many coyotes come together as males wish and hope to get a mating partner.