There are carnivorous plants that consume animals for chemical elements that are lacking in their soil or water environment. Instead of enveloping prey, which limits the size of the prey that can be captured, what if an alien plant had spines that were freakishly poisonous? Something of the order of potassium cyanide.
Bump into the plant and you're screwed. You drop, rot, and fertilize them.
If their poison of choice would work on humans effectively would be blind chance.