Annie Lowrey fingers cultural forces:

In the past year or two, scads of blogs have committed themselves to exposing law school as a "scam," and the New York Times and Wall Street Journal have devoted thousands of words to telling readers why law school is a bad, bad idea if you do not actually want to be a lawyer. Look to any of a dozen blogs or news sites to explain how wages for legal workers might continue to fall, as automation takes over rote tasks and businesses increasingly refuse to pay obscenely high per-hour fees. Wandering further into the realm of anecdata, virtually every young lawyer or law student I know would love to talk my ear off about the worrisome employment prospects for new legal professionals.

Once the conventional wisdom has spotted a bubblewhether in housing or gold or anything elseit tends to burst.