Gandharvas under the leadership of Tumvuru and Chitrasena had valid reason to support Arjun.
During Kurukshetra battle most of the celestial groups supported Arjuna and Gandharvas are one among them.
auspicious animals and birds were on Partha’s side, wishing for his victory. The Vasus, the Maruts, the Sadhyas, the Rudras, the Vishvas/^ the Ashvins, Agni, Indra, the moon god, the wind god and the ten directions were on Dhananjaya’s side, while the Adityas were on Kama’s side. The gods, together with the large number of ancestors, were on Arjuna’s side. Yama, Vaishravana' 7 ^ an( j Vamna were also with Arjuna. The gods, the brahmanas, the kings and the large number of rishis were with Pandava. O king! The gandharvas, with Tumburu at the head, were with Arjuna. The descendants of Prava and Muni, the large numbers of gandharvas and apsaras/^ wolves, predatory beasts, animals, birds, large numbers of chariots, different forms of clouds and winds, and sages assembled there to witness the clash between Kama and Arjuna. O great king! The gods, the danavas, the gandharvas, the nagas, the yakshas, birds, maharshis learned in the Vedas, the ancestors who thrive on svadha oblations, ^4 austerities, learning and the herbs, in many different forms and attires, took up their
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prominent Gandharvas like Chitrasena and Tumvuru were mentioned as Arjuna’s friends. Chitrasena taught him music and dance while both he and Tumvuru were present at Indraprastha as per narration.
Chitrasena fought with Arjuna during Ghoshayatra war but got defeated by him .Indra said the same
The Pandava is your beloved friend and your student
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Both Tumvuru and Chitrasena resided at Indraprastha because of their friendship with Arjuna.
Samba and Yuyudhana Satyaki.^4 O ruler of the earth! These and many other kings were there. Dhananjaya’s eternal friend Tumbaru was there, Chitrasena with his advisers and many other gandharvas and apsaras, skilled in singing and music, knowledgeable in the beating of tala' andkinnaras, who were excellent pupils of laya. At Tumbaru’s command, those learned ones sang in celestial tones, as was laid down in the rules, and pleasured and paid homage to Pandu’s sons and the rishis who were assembled in the sabha, rigid in their vows and devoted to the truth. They paid homage to Yudhishthira the way the gods do so to Brahma in heaven.’
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Thus Tumvuru,Chitrasena and many other Gandharvas were in good terms with Arjuna and therefore naturally supported him in his final battle with Karna. Even Angaraparna Gandharva also was a friend of Arjuna after facing defeat through Agneyastra .
With prominent Gandharvas being close to Arjuna,it was not a surprise to see them chose him over Karna.