What I originally read was that Imprelis herbicide, which was marketed as an environmentally friendly herbicide for professional use (landscape services, etc), was suspected in the deaths of thousands of Norway Spruces, eastern white pines, and other trees on lawns and golf courses across the country. (Source)
I just got the chance to look into it further and have discovered that DuPont, the manufacturer of Imprelis, has halted sales of the product and is planning a product return and refund program. Additionally, they have setup an entire website, called Imprelis Facts, to answer questions about the product.
If you utilize a landscape service to maintain your lawn, you might want to ask them if they use Imprelis.
I tried to dig a little deeper and find out why it was first thought to be environmentally friendly. I only found a vague mention of it being "less" toxic to mammals.
I also thought it was interesting that, even though it was marketed as environmentally friendly, companies that applied the product were instructed to provide this notice to all of their customers:
Today we have treated your lawn with an innovative weed control product from DuPont. The product label requires that you do not use grass clippings from areas treated with Imprelis for mulching or compost, or allow for collection to composting facilities. Grass clippings must either be left on the treated area, or, if allowed by local yard waste regulations, disposed of in the trash.
As usual, my recommendation is to stay away from herbicides and other lawn chemicals whenever you can, even if they are listed as being more environmentally friendly.
Changing Your Relationship With Weeds
Please Don't Poison My Planet
Beware of Second Hand Herbicides