When sharks are angry they "arch" their backs and turn the pectoral fins down. This is known as the threatening behavior. They do it just before an attack. I've seen it before, when I had a bag of fish hanging behind me, and believe me, you don't want to see it.
I've attended numerous shark feeds, and the only time I've seen the arched back and downturned pectoral fins was in Kavieng, PNG, when a few of the silvertips assumed that posture as they came in for chunks of fish. In all other instances, they just swim in and grab some fish/chum, but didn't act like they were having to be "aggressive" to do so. In most cases, sharks are aware they are the apex predators in that situation, and don't seem to feel a need to prove it... Of course, that's just my experience.