Ya, you're right. Someone probably stopped and let them out for a pee, and simply forgot them. Or, someone saw them, realized they were abandoned, so stopped on the highway, left them some spare dog food in the ditch, and then left them there. Hmm. Ya, that coulda happened...:)
Or, as my money is bet on, there is an irresponsible ex dog keeper in the picture here somewhere....seriously...

not saying one way or another.. if the story is real and most likly is, then ya they deserve to get what they got comming, but I have seen it alot.. especialy when I lived in the country that people notice dogs hanging around an area.. see them later and feel sorry for them and give them food. then dogs being dogs they hang around and some one else gives them food.. and so on and so on... I hate it when people feed stray dogs myself as they hang around from that point on.

Steve