what the heck was this about?

well, it was even shown in the movie that it was mysterious and they dont know the reason. they hinted that it might be due to some chemicals released by the plants. but again the question arises plants are present everywhere then why not the whole earth suffered altogether... Anyways, its after all just a movie, dont ya worrie... I didnt like the ending of this movie :p i was expecting much more thriller ;)

Yeah, it was kind of like a global warming thing. The vegetation is getting back at us for upsetting the earth. The premise could have been more powerful with a better script. Instead it came across as kind of trite and preachy. M. Night isn't one of my favorite writers, but he does know how to direct well.