The happening came out when global warming first started to hit the news media heavily. If you recall Wall-e also came out around the same time. These were just two of the movies that year that revolved around global warming and its ramifications.
I thought the Happening was great and I think people who disagree are probably those people that didn't and do not take global warming and environmentalism seriously.
This isn't even a paranormal idea. It is a super-natural thing- it is completely realistic. There ARE plants that release pollen and sap, etc to control the life around them. They draw bugs or smaller animals near to protect themselves from their predators or secrete poison to kill them.
The movie didn't say, Trees talked and said let's kill everyone. And, I think that is another thing people just assumed. i heard a lot of people commenting, "Trees killing people is stupid." But it's not.
What was Happening was a biological defense mechanism of nature.