why are there so many dead cows wandering the roads?

The cows are wandering the roads because their owners are in jail.

Cheever: There be so many cows wanderin’ the highroads, now their masters are in the jails, and much disagreement who they will belong to now. I know Mr. Parris be arguin’ with farmers all yesterday—there is great contention, sir, about the cows. Contention make him weep, sir; it were always a man that weep for contention. He turns, as do hathorne and danforth hearing someone coming up the corridor. danforth raises his head as parris enters. He is gaunt, frightened, and sweating in his greatcoat.