Malbim: This is a decree with a Shevu'ah, which is not torn, because My flock was la'Vaz (plundered), without a shepherd. An armed robber or evil Chayah [that takes or tears animals] is Ones, but if [the shepherd] entered the city when it is not normal to do so, the beginning was negligence and the end was Ones; he is liable. And especially here [he is liable], for the flock was la'Vaz and Chayos ate them because there was no shepherd. It is as if he took them to a place of armed robber and Chayos; he is liable (Bava Metzi'a 94a).

Malbim: [The shepherds] should have hired other shepherds [to fend off the Chayah] with sticks, especially since they are like paid Shomerim.

Rashi: They graze themselves. Malbim ? they should have grazed My flock! Do not say that they ceased guarding, and did not want to be shepherds any more ? they grazed (took from the flock for) themselves!