If this is one of the random images from the PD's WEB site Home Page with the scenarion you note, I raised the same question approx 25 years ago......
Eddy Mathews answered that it is a fast flowing tide passing through a relatively narrow mouth that gives the impression of a reversal of directions
It was baffeling for me to comprehend, Eddys comment made sense, it was just that living here on the Eastern seaboard of OZ, I had never see such tidal effect
Derek
[PS...in earlier years I had retained an .jpeg folder of the Home Page paddler images, it was however lost in a computer crash]