A pre-hospital team is called to a 55-year-old male who has been stung by multiple bees (estimated 30+ stings) while trimming a hedge near a hive. He has no history of bee allergy. He is tachycardic (125 bpm), hypotensive (85/60 mmHg), nauseated, and has widespread urticaria. His symptoms are not responding fully to IM adrenaline. What specific phenomenon is occurring?