My Glen of Imaal sometimes gets this. The vet, his breeder and my not so experienced mother put it down to 'excitement'.

First poop - 100% normal. Second, usually normal, sometimes a bit 'wet' at the end. Any subsequent, a high chance of 'runny'. Only ever on walks (never at home), and usually only on unusual walks - it is rare on our normal, every day walks, just week-end walks that we drive to.

Personally? I think it is stress and he isn't quite the confident, laid back dog he seems.

It is a b'stard to clean up - I always take kitchen towel with me. But doesn't seem to bother him. We live with it.