Among other suspicious behavior, Sanford's accusers noted that she had partaken in "a bottle of sack" (liquor) on the Hartford town green. Avery guesses that perhaps such behavior ruffled the stodgy community.
Chimney rage is another possibility. Mary's husband, Andrew Sanford, held the post of "chimney viewer" - one who makes sure everyone's chimneys are clean and in good condition. One could have made plenty of enemies with such a job. [Link]