what later event in the act do their arrests foreshadow?

Rebecca Nurse is accused of witchcraft. The Putnam’s, Mrs. Putnam in particular, feels that Rebecca has something to do with all her dead babies. Giles Corey casually mentions that his wife reads books that are not the bible. He calls them "strange books". This is enough to ignite and foreshadow the ridiculous hysteria that will plague Salem.