Bonehart plague carrier

[Krfan]

Can someone tell me what triggers plague carrier?

The wiki says there must be 4 enemies in a row but that's not true( he used it on two abominations).

Reload time can't be the only factor because sometimes he uses them within, I dunno, approximately 20 seconds of each other. Other times he uses the second plague carrier after 5 disease novas. I think there has to be a certain amount of health the enemies have that trigger it but I'm not sure.