Resistance is due to genetic mutations in the coccidia allowing them to escape the anticoccidial activity of some of the drugs. Thus, there are 2 factors contributing to resistance: the genetic changes and the selection pressure by the anticoccidial.
Giving resistant coccidia a selective advantage over the sensitive coccidia in the population of coccidia in the broiler facility. This allows the resistant coccidia to become the predominant coccidia in the population within the broiler facility.
The probability of resistance is dependent on the mode of action of the anticoccidial.