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The key drivers of the decline appear to be the perception that the settlements damage Israel's ties with the United States and may lead to international boycotts and isolation (both very correct perceptions), as well as the growth of settler violence directed at the IDF (the Israeli army).
Notably, the poll itself seems to have been commissioned by a settler group, Samaria and Jordan Valley Research and Development Center. The problem with the story is that it gives various results from this poll but doesn't give any trend data going back to 2009. In other words, X% of Israelis supported settlers in 2009 and that number has now dropped Y points. Unfortunately, I can't find any reference to the poll outside of Haaretz's reporting. So I can't get these details.
This isn't terribly surprising since it was it seems to have just been released and I'm limited to searching for information published in English. If any readers have more information on the poll, please drop me a line by email. It's very difficult to know the significance of the poll without more details.