Some fish are very territorial, especially in spawning season. Yes, this is relatively common.

All of my scuba diving is conducted on the West Coast, from British Columbia to Orange County, CA. In the Puget Sound (WA), if you invade the territory of a male cabezon (a type of sculpin) while it's guarding eggs, he will defend his territory -usually by ramming the diver - and that 25-lb fish will get your attention!
If you encroach on a mature female ling cod (type of greenling), she will bite. Ling cod grow to 4.5-5 feet in the Puget Sound, and their mouths are equipped with 18 sharp teeth. That will teach you a little respect for her territory.
In California, the state marine fish is a garibaldi, a brightly colored species of damselfish which grows to 12-14 inches. Invade its territory and it won't care if you're a 200-lb diver, it will peck at you.
None of these present a threat to life, but they are not intimidated and will not hesitate to protect their territory. I haven't dived the Hawaiian Islands, but I'm not surprised the snorkeler/diver was bitten on the ankle. It happens.

I'd be careful of what kind of conk you put in your pocket. I found a very nice looking 2 inch conk and put in my pocket and continued diving gathering other shells. It was'nt untill I arrived mainland I remembered about Hawaii having poisonous conks. After checking, sure enough I had the conk that can kill in minutes.

They are usually pretty docile off those islands. You weren't dribbling out peas from the sandwich bags they sell at the beach were you? People feed them with the peas and they do get in a feeding frenzy and will nip your fingers, anklles, etc.