The expedition of Ukasha bin Al-Mihsan Al-Asadi to Udhrah and Baliy (or Balli) doesn’t tell us why the Prophet (p) did so. The report cited only states that he sent an expedition. Doesn’t give us further details than this.
Kitab al-tabaqat al-kabir – Ibn Sa’d:
“THE SARIYYAH OF ‘UKKASHAH IBN MIHSAN AL- ASADI AGAINST AL-JINAB THE TERRITORY OF THE ‘ UDHRAH AND THE BALLl
Then (occurred) the sariyyah of ‘Ukkasha Ibn Mihsan al-Asadi against al-Jinab, the territory of the ‘Udhrah and Balli in the month of Rabi’ al-Akhar of the ninth year, from the hijrah of the Apostle of Allah, may Allah bless him…” 
We may say, the Prophet (p) most probably sent Ukasha on this expedition because of the on going hostilities and or war these people were doing against the Muslims. The Prophet (p) in a Hadith states in clear words, praying to God, that the fighting he engages in against enemies at the time, be aimed only at those who oppress and are hostile to the Muslims:
“O GOD! let us enjoy our hearing, our sight and our power as long as You keep us alive and make our heirs from our own offspring, and make our revenge RESTRICTED TO THOSE WHO OPRESS US, and support us AGAINST THOSE WHO ARE HOSTILE TO US let no misfortune afflict our Deen; let not worldly affairs be our principal concern, or the ultimate limit of our knowledge, and let not those rule over us who do not show mercy to us.” (Riyad as-Salihin Book 5, Hadith 834)
These clear words of the Prophet (p) show is that any of the fighting he did in his life-time were only against those who were hostile to the Muslim community.
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 Kitab al-tabaqat al-kabir,By Ibn Sa’d,Volume 2, page 203