Arjuna has cut-off their trunks. These are huge chariot like Gandharva city and these beheaded kings are falling down from them, as the heaven dwellers fall from air-craft down when their account of noble deeds is squared up.
गन्धर्वनगराकारा रथा वा ते नरेश्वराः। विमानादिव पुण्यान्ते स्वर्गिणे निपतन्त्यमी॥