Beak Blast is a decreased priority move. The user charges up at the beginning of the turn and executes Beak Blast at -3 priority. All who make contact with the user during the charging phase are burned.
The charging phase occurs whether or not the user's status would prevent it from executing Beak Blast. For example, if the user is confused, the confusion check is performed before the strike, not the initial charge.
Pokémon with Bulletproof are immune to Beak Blast, but are not immune to the burn.