It seems to me that the art of making properly rounded ski turns has been lost by much of the skiing public. Other PSIA Level 3 certs I talked with have confirmed it: they have never spent so much of their teaching time working with clients to make round turns.
I attribute this, at least in part, to the increase in the amount of tip and tail rocker on the skis being used by everyone. Rocker makes it easier to twist skis quickly and the result is that all the turning force comes right at the apex of the turn.
Our self-preservation instincts push us to get around that corner as quickly as possible so the response is to twist to get the started and finished post haste.