At this spot the dolphins will bother the fishermen by taking the fish off the hook - it actually happened to me... I was mad lol. The fishermen know better then to bother the dolphins there - that would be asking for trouble. The location of this place is also inside a state park =] It is really pretty.
These dolphins while they do chase the fish and the fish obviously run from them - the fish do not run from the pier they still hang around the pier and you can still catch plenty of stuff even with the dolphins present.
As for why they are jumping there could be for a number of reasons. It could be a socialization thing, it could be because things are attached to their skin that they do not like (remoras attach to dolphins, not just sharks and spinner dolphins for example are believed to spin with the physical force that they do to fling the remora off), It could be for play. I am not really sure why they are doing it though.