Hamlet, as part of his plan bring down Claudius and Gertrude, feels it necessary to make everyone believe he is insane, even poor Ophelia.
Instead of the tender lover she has come to know, Hamlet attacks her with vile insults and words. He accuses her of the very falseness his mother has demonstrated. In 3.1, he rails:
"...God hath given you one face, and you
make yourself another. You jig and amble,
and you (list); your nickname God's creturea dn make
your wantoness (your) ignorance. Go to, I'll no
more on 't. It hath made me mad. I say we will have no more marriage. Those that are married, already,
all but one, shall live. The rest shall keep as they are.
To a nunnery, go" (3.1.155-162).
His cruelness sends Ophelia over the edge. Without her brother Laertes to turn to, and Hamlet's scorn, Ophelia begins her descent. More tragedy for the young girl will follow, as Hamlet will murder her father, Polonius.
Hmm. This act only? I think the seeds are planted in other acts as well, but if we limit ourselves to just this act, I'd look at the following:
Hamlet is sexual towards her, rather than just loving.
She can't deal with this direct address, and turns away (showing denial of reality).
She seems fairly easily upset by things.
She is confused by the players' play, or claims to be; either one would show vulnerability to the court politics.
She sees the king upset by the players' play, which should upset her emotionally. Once her father is killed, she might put together that he was serving a killer.