A very good question!
For full disclosure, I do not think she fell into the river, I lean towards a third party intervention. Given that stance, I'm interested in what would have caused her to abandon phone and dog, and leave, e.g. through the kissing gate. I'm not sure where it was reported, but my impression was that the dog was found running between the bench and the kissing gate. Looking at the report on GMB yesterday (Thursday), the metal kissing gate is very close to the back of the bench, and leads to a tree/shrub lined footpath. Perhaps, at the crucial time, Willow was a little way away from the bench/gate and saw NB leave via the gate, so was naturally drawn to that area.
So why did she leave? Well, I can imagine if a third party came down the path to the gate and told her, for instance, they had come from the school and that one of her children had been taken ill/ had an accident, she would think of nothing other than to get to the child, forgetting everything else. I cannot imagine any other reason why she would just leave. Given the timings published by LE (last sighting 09:10, phone found 09:33), 20 minutes seems ample to lure her away. What happened therafter is not for me to speculate.
MOO