I don't know anyone that has spent any time at all near running water that hasn't been surprised by a bear at one time or another in their life. They are amazingly quiet when walking on their well worn paths and only really make a racket when going off trail. I was doing some prospecting with a new metal detector that a buddy sent home with me to try out, I had ear phones on so I could hear the pings and pongs, came around a bend in the crick and stood ten or twelve feet from a good sized black bear equally intent on spawning Kokanee, we both went our seperate ways instantly. I was always super careful when going over the breaks into the neighbor watershed because we had a strain of grizzly there on the Granby river, a small pocket of bears that were smaller than your standard grizzly...they still look huge. That was the only place I ever met another hiker in Canada that carried a firearm, he had a 12gauge slug gun over his should, said "Bears scare me."
I could easily see how a guy could get caught up in fishing and come around a small bend catch a bear off guard with the wind in his face, bears just do what comes natural to them and defending a kill after getting surprised is natural...sure beats the hell out of dying in your own mess in a nursing home.
