what causes Mary Warren to break down?

Asked and answered.

1) Mary feels guilty about the poppet, and the fact it brought suspicion against Elizabeth.

2) Mary is afraid of Abigail.

3) Abigail pretends to see a yellow bird attacking her.

4) Mary is asked to faint but is unable to. This leads Parris to say she is lying.

In the end, Mary proves to be such a poor witness that she ends up accusing John Proctor of witchcraft.