why has martha corey been accused?

If you recall, back in act !, Giles Corey makes an off handed comment to Hale, more out of jest than anything else. He says that his wife stays up late reading "strange books" which makes it hard for him to get to sleep. In Act 2 Martha is accused of witchcraft. Apparently she sold some cows to a man and the cows died. This combined with the whole reading thing was enough to put her on trial.