Yep, all of the above. Some dogs eat grass for any number of reasons, upset stomach, nerves, need nutrients (although grass would not provide any nutrition anymore than eating whole veggies. Their digestive tracts are not long enough to break down plant matter), something to do.... some dogs just like to graze, or get at the roots and grubs under the grass.
On weed killer. Don't worry about it AFTER it has dried. Of course don't let your pets on weed/grub killers and fertilizer until it either dries or gets absorbed into the ground, some stuff says 48 hours, but once that happens you are safe. Your pet would never eat enough to cause it harm. I got this info from several manufacturers and use weed/grass killer without ill effect and one of my own poochies is a grazer.