Whoops, I didn't clarify that!

Arthur had been around for the Wise Snake and as a result, developed a fear of large reptiles/snakes in particular. The sight of a dinosaur walking down the street struck him with an incredible feeling of fear, echoing his past life's ability to be able to read in exceptionally dangerous situations. Basically, it's the immediate fear he's facing and the just as sudden realization that he isn't actually in danger that would be the trigger.