Because Agamennon takes prisoner a priestess (Criseida,or Criseis en English) of Apollo's cult. Her father asks for her liberation for ramson but Agamennon refuses. The father prays to Apollo who is enraged to know about it and punishes Agamennon hubrys sending "arrows of plague" to the achaean camp.
All this happens in Homer's Iliad, the story about the troyan war.
I believe this is the right answer but please do more research on this topic to make sure.
If I got it right Hope I helped!
