You’ve probably done nothing wrong. Pretty much all chickens carry coccidia. It gets spread around, but does not cause problems unless the levels get out of hand. The medicated starter is supposed to act as a preventive until immune systems are built up, but sometimes it’s just not enough. Healthy chickens are able to build up immunity to prevent this, but sometimes immune systems are compromised due to stress, illness, etc. Chicks and young birds that have not had time develop immunity are also at a greater risk. Something as simple as a change in weather can bring it on. The best thing to do is start treating ASAP and keep everything as clean as possible during this time as they’ll be spreading it. It will not hurt the birds (even those not showing symptoms) to have corid in the water. Basically, if one has it, they all have it or are at risk of having it so it’s best to treat the whole flock. The good news is once they’ve had coccidiosis and have successfully gotten through it, they’re typically immune from having it again.