Properly store food, including pet food, so that bears cannot smell or reach it. Don't keep food in your tent - not even a chocolate bar. Pack out all garbage. Females defending cubs. Bears defending a fresh kill.
Bears can climb trees, although black bears are better tree-climbers than grizzly bears.
Bears have excellent senses of smell and hearing, and better sight than many people believe. Bears are strong. They can tear cars apart looking for food. Intrusion into this space is considered a threat and may provide an attack. Bears aggressively defend their food.