I think Portman has gotten a lot more critical in recent years than she used to be. After all, she studied under Alan Dershowitz (*shudders*) at Harvard. But stuff like refusing to attend the Genesis Prize ceremony because of what she correctly called "atrocities", flat-out calling Netanyahu a racist and now reposting unambiguously pro-Palestine posts on Instagram shows a major change.
I think its representative of how many American Jews (especially young American Jews) feel increasingly disconnected and alienated from Israel. As Israeli politics have shifted further and further to the right, and overt racism has been normalised in public discourse, most American Jews (who are overwhelmingly left-liberal) feel they cannot defend it anymore.
I think the election of Trump hastened this as well, given that he was the most overtly pro-Israel candidate ever (truly an impressive feat) and yet 75% of American Jews didn't vote for him.