One thing to remember is that they do go through another "fear" stage at 7-8mths old, although what you have described doesn't seem to be fear based, unless it is happening approximately either the same timeframe into your walk, and or, at the same geographic locale.
As you might know I have big boy Magic, and at approx. 7.5 mths old, all of a sudden and he had come to my clinic every day since he was 8wks old, balked at walking down Yonge Street, a very busy street in downtown Toronto. I mean Friday he was fine, Monday he was not!
When I went to my trainers that Wednesday and explained what had been happening, she said Gail, he is going through his last fear stage; can you walk him another way into your clinic for now? Of course I could and did. He was fine with the different approach; one week later we did our original walk to the clinic and then he was just fine!