Using this definition, can you summarize why hyperventilation is considered dangerous?

Hyperventilation does not increase the amount of oxygen available but artificially decreases carbon dioxide (CO2) levels in blood and lungs. 
Because the primary urge to breathe is triggered by rising carbon dioxide levels, it gives false impression that one does not need to breath. This can lead to sudden lost of consciousness. If this happens underwater, there is a considerable risk of drowning. 
This is why hyperventilation is considered as dangerous and should be avoided.