Expedition of Zaid ibn Haritha took place because some robbers attacked the Prophet’s (p) envoy. Hence, these criminals had to be dealt with or they would continue with their criminal activities.
Kitab al-tabaqat al-kabir – Ibn Sa’d:
“THE SARIYYAH OF ZAYD IBN HARITHAH AGAINST HISMA
Dihyah Ibn Khalifah al-Kalbi came back from (the Ceasar) who had entertained him and had given him a robe. Al-Hunayd Ibn Arid and his son Arid Ibn al-Hunayd along with the people of Judham encountered him at Hisma. They waylaid him and did not leave anything except used clothes with him.” 
“Ibn Humayd-Salamah-Ibn Ishaq – one who could be trusted from the men of Judham who were knowledgeable: When Rifa’ah b. Zayd came [to his people] with the Messenger of God’s letter inviting them to Islam, they responded. Soon [the following incident happened]. When Dihyah b. Khalifah al-Kalbi, who was returning from Caesar, the Byzantine King, [to whom he] was sent by the Messenger of God with some of his merchandise, reached a valley called Shanar, al-Hunayd b. Us and his son is b. al-Hunayd al-Dulay’I, a clan of Judham, ATTACKED HIM AND SEIZED EVERYTHING THAT HE HAD. This news reached some people of the Banu Dubayb, the kinsfolk of Rifa’ah who had embraced Islam. They responded and went after al-Hunayd and his son; al-Nu’man b. Abi’Ji’al of the Banu Dubayb was among them. When they encountered them, a skirmish followed. … The Messenger of God sent Zayd b. Harithah with a force against them, which was what provoked the raid on Judham. All of Ghatafan of Judham, Wa’il, those of Salaman, and Sa’d b. Hudhaym had set out when Rifa’ah b. Zayd came with the Messenger of God’s letter and camped in Harrah [the volcanic tract] of Rajla, while Rifa’ah b. Zayd was in Kurra Rabbah with some people of the Banu al-Dubayb, now knowing [what had happened]. …” 
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 Kitab al-tabaqat al-kabir , Ibn Sa’d, volume 2, page 108
 The History of al-Tabari: The Last Years of the Prophet: the Formation of the State (Translated: Ismail K. Poonawala), volume 9, page 99 – 101