Broken Rocker Arm on 2004 Chrysler Pacifica

I'm looking for ideas on what might have caused a rocker arm to break! Dealership replaced arm & camshaft (under warranty), but doesn't have a reasonable idea of what might have caused the problem.

Asked by for the 2004 Chrysler Pacifica
sometimes there will be a hairline crack in parts that you can't see, thats why racing engine builder magnaflux parts before installing to make sure the parts don't have cracks so it could have been there the whole time and suddenly failed
1 more answer , 1 more comment
I have a 2006 Pacifica and I had to replace 2 rocker arms, do not know how they went bad, just started knocking on a long trip that I just took. Parts aren't cheap but the labor cost is out the window, don't want to have to replace many of these!
I have a 2004 Pacifica and they just replaced the cylinder head gasket and the cylinder head assembly due to the rocker arm breaking. Total was $3500 with was it fixed in other automobiles?