David Paul Kuhn draws attention to the politicization of the Supreme Court. I agree with him that the trend toward single-vote majorities is poisonous. He sees it as an expression of wider political polarization in the US. I am less sure that this is the cause, or at any rate the only cause.
An important factor is increasing remoteness from the amended text. With time, society changes, and the gap between justices who believe in fidelity to the original document and those who believe in a "living constitution"--equally defensible positions, regardless of one's politics--gets wider. Over time, there will be more good-faith dispute about what a relatively static written constitution still means. Even without greater polarization, you would expect more disagreement, and more narrow majorities.
Other factors are probably at work. One is mere familiarity. Failure to achieve consensus and the divided courts this produces are habit-forming. Alignments, including political alignments, come to seem more permissible. In due course voting your politics becomes not just acceptable, but obligatory (because the other justices are going to, even if you don't). When you look at the process for appointing the justices, and the way they divide on politically sensitive issues, it is surprising that they any longer pretend to be politically neutral. Is anybody--anybody at all--taken in by this? Already, it is a purely ritual affirmation. I wonder how much longer this hypocrisy will be deemed necessary.