The commentator indicates the name it got as it belongs to the मण्डूक-शाखा of the अथर्ववेद just like the name तैत्तिरीय उपनिषत् attached to तैत्तिरीय-शाखा of यजुर्वेद. This is according to Upanishadhmayogin.
अथर्ववेदप्रविभक्त-मण्डूकशाखा-मस्तक०
i.e. the Vedanta portion of मण्डुक recension of Artharvaveda.
Thanks for the different views provided in this conncetion.