Well, one hypothesis is that scientists are acting in a self-interested manner. Liberals and the Democratic party seem much more willing to raise and spend public money to fund scientific research than conservatives and republicans. Indeed, while the survey did not specifically ask about reasons for the scientists' political views, the data from the survey support this hypothesis.
1) 84% of scientists identify a government funding source as one of their most important sources of funding (only 50% identify a non-government source). This holds true even for scientists working in industry (64% of whom identify a government source as a most important source of funding). Therefore, a great majority of scientists rely on government funding.
2) 46% of scientists say that lack of funding for basic research is a very serious impediment to high-quality research (only 12% say lack of funding is not too serious or not at all serious). Thus, many scientists would likely support politicians who would increase spending on scientific research.
3) In a poll of the pubic, 51% of those who identified as Democrats supported increasing federal spending on scientific research (versus 25% of Republicans). Furthermore, 21% of Republicans supported decreasing federal spending on scientific research (versus 8% of Democrats). Thus, these data support the notion that the Democratic Party is more likely to support federal spending on science.
Other hypotheses might include the finding that only 33% of scientists believe in God while 84% of the public does.
As a scientists myself, I will admit that self-interest is one reason why I identify as liberal and support the Democratic party. However, I would not classify self-interest as the main reason. Many of the important issues for the Democratic Party are also important for me, including increasing the role for the public sector in healthcare and curbing greenhouse gas emissions via increased regulation. I also am in support of liberal social policies (pro-choice, support for gay marriage, tighter gun control laws, disagreement with abstinence-only sex education, etc.). So, while self-interest may be part of the answer, it is not the whole story.
Perhaps science and academia in general (63% of the scientists surveyed were in academia)--a field with relatively low income for the amount of education necessary when compared to careers in medicine, law, business, finance, etc.--attracts those with more left-leaning tendencies. Indeed, only 33% of the scientists surveyed identified a financially rewarding career as a very or somewhat important reason for entering science. However, I have no data to support my claim that those with less financial motivation are more likely to identify as liberal.
Regardless of the reason, a president who calls for America to restore science to it's rightful place in society (a statement met with cheers as I watched the inauguration with an auditorium full of chemists) is sure to win over scientists all over the country.