Neither "lurch forward" nor "walk like a duck" when practicing the Classical Tai Chi basic walking. Video demonstration
"That (keep your weight back) while walking, takes a lot of patience" Hwa Laoshi
If you do not keep your weight back THEN place your entire foot on the ground, you not only lurch forward but succumb to the "rocking chair" effect. One's weight is actually rocking forward as the weight shifts, makes it easy for an oppone nt to pull you over.
Also, "they do not stand up" in regard to keeping a consistent knee bend. Constantly standing up results in a kind of "duck walk" wherein the body is bobbing up and down.
Imagine each of those people keeping their head level with a line drawn across that mirror. While watching their reflection in the mirror their head would not rise above, nor below that "line" as they walk.