My guess is that it is out of hunger. Chapter 17, Part 3, 11:40, Skittles says that they eat anything, even each other if they have to. He may have been only talking about the behemoths, but I think he meant all of them.
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Tanya and Saul saw one eat another one near Ground Zero--the one that was sniffing around their car, truck, whatever.
Most Behemoths seem anti-social; Skittles said the regulars avoided them because they'd eat other zombies from the get-go. Maybe it's because of who they were before (murderers, etc). I've also gotten the impression that they have to be really hungry before they'd eat each other. Maybe that's how the plague would eventually burn itself out--without Ink's help they'd eat each other until there were a manageable number,
To the tune of "Food, Glorious Food" from Oliver
Brains! Wonderful brains!
Hot blood and intestines!
We eat the remains--
Cold liver without Heinz!
Please give me some heart and spleen!
Who's next is the question?
Rich gentlemen or hoi polloi?
A rhe-tor-ical question!