This is simply a pop-reference to the ordinary ‘but-for’ test.
In Wallace v Kam  HCA 19 the High Court considered a ‘failure to warn’ negligence case involving a doctor who did not warn the patient of all of the risks of a particular procedure. The patient was injured by a reason other than the risks which the doctor did not warn.
In the judgment the Court said:
A useful example, often repeated, is that of a mountaineer who is negligently advised by a doctor that his knee is fit to make a difficult climb and who then makes the climb, which he would not have made if properly advised about his knee, only to be injured in an avalanche. His injury is a “foreseeable consequence of mountaineering but has nothing to do with his knee”.
The example was first used in Banque Bruxelles Lambert SA v Eagle Star Insurance Co Ltd  UKHL 10;  AC 191 at 213.