David Paul Kuhn draws attention to the politicisation 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 polarisation 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 polarisation, you would expect more disagreement, and more narrow majorities.