How would I describe how a triffid walks? Why do triffids lurk near their fallen victims ?

The triffids sway rather than walk.

"I was permitted to see a few triffids sway across the screen to the kind of accompaniment supposed to be on the level of the great movie-going public:"

"The jungle natives took to carrying long, light poles mounted with hooked knives, which they used effectively if they could get their blows in first- but not at all if the triffid had a chance to sway forward and increase its range by an unexpected four or five feet."

The triffids lurked near their victims so they could feed on their flesh after they began to decompose.

"The reason for that became clear only when it was shown that they fed upon flesh as well as upon insects. The stinging tendril did not have the muscular power to tear firm flesh, but it had strength enough to pull shreds from a decomposing body and lift them to the cup on its stem."