Humphrey has phases of doing the same and I‘ve never really figured it out other than on occasions where I did give, because I thought he was in pain or whatever, he‘d turn around and walk normally, so I‘ve come to the conclusion he‘s just trying it on!

I usually just carry on walking now and once he realises I‘m not turning round, he‘ll come trotting up behind me.
Do you time the walks around meal times? If we‘re running late, Humph‘s always reluctant to go out because he seems to think I might have forgotten to feed him and when we do, he get back quick enough!
If you‘ve discounted any health issues, I‘d just put it down to cockerism

- maybe try a few games to jolly her along and distract her from whatever it is that‘s spooking her, maybe scatter some treats up ahead so she has to keep moving to sniff them out?
Julie has a good point, Humphrey doesn’t like the heat either and even when its cooled down, the ground is still warm.
Best of luck,