Hmmm. Well, Murphy develops weird, irrational fears all the time. He was terrified of the back a while back. I finally figured out that the neighbor's curtains were blowing out the window and he was scared for some reason
Murphy developed a fear of his food bowl last year. I couldn't figure it out at all! A few weeks later, I noticed one of his tags in between the crates with the jump ring all stretched out. I figure he got his tag caught on his crate while he was eating = fear of the bowl.
My point is that sometimes dogs get scared of weird things and make strange associations that we don't even think about. Could be a scary sound, could have been stepping on something sharp out there...you may or may not figure it out.
I agree with the above. Take it easy for a while. Make the back yard super fun. Murphy is very food motivated, so even with his fear of the curtains, he'd go out to hunt for kibble in the grass.