Explanation From Wiki:

https://en.wikipedia.org/wiki/Cumans#Settlement_on_the_Hungarian_plain

In 1238, after Mongol attacks on Cumania, King Béla IV of Hungary offered refuge to the remainder of the Cuman people under their leader Khan Köten, who in turn vowed to convert his 40,000 families to Christianity. King Béla hoped to use the new subjects as auxiliary troops against the Mongols, who were already threatening Hungary. The Cumans were joined by the Iranian Jasz people, who had been living with the Cumans.[21]: 44  Batu Khan of the Mongols then ordered Bela to stop giving refuge to the Cumans and made a particular point that if attacked the Cumans could easily run away, for they were skilled horseman, but not so for the Hungarians, who were a sedentary nation and had no such luxury. Bela rejected this ultimatum.

Around December 1240, news came that the Mongols were advancing towards Hungary. King Bela then installed front line defenses at the Carpathian Mountains, after which he returned to Buda and called a council of war and ordered unity against the Mongols. The opposite happened, however, as many of the barons were hostile towards the Cumans. The Hungarian barons noted that there were Cumans in the Mongol armies, but they did not realize that this was because they were conscripted into it and had no say in the matter. In particular the barons did not trust Köten, despite the fact that the Mongols had attacked his people for nearly 20 years. This chaos pushed Bela into a corner; feeling he needed to show his strength and keep the rebellious barons on his side, he ordered Köten to be placed under house arrest. This did not placate the barons and had an opposite effect of erroneously seeming to admit that the King harboured doubts about Köten, too. This angered the Cumans, who were far from happy about the actions taken against them, and who had done nothing to invite such actions and hated the Mongols.

News arrived on 10 March that the Mongols had attacked the Hungarian defenses at the Carpathian passes. This prompted Bela to send a letter to Duke Frederick of Austria asking for help. Frederick had previously wanted Bela’s throne, but Bela responded by amassing a vast army and marching to the gates of Vienna, which forced Frederick to step back. On 14 March, news had arrived that the Carpathian defense forces were defeated by the Mongols. Ironically, given the suspicion of the Cumans, they were the only ones who seemed willing to fight the Mongols, the memory of the fate that had befallen them on the steppes still being fresh in their minds. By this time Bela had lost control of his army and many towns were destroyed. Soon thereafter Frederick arrived, and, wishing to harm the country’s defense (in revenge to Bela), he stirred up further feelings against the Cumans.

After crushing defeats and facing complete collapse, the Hungarians engaged in a suicidal betrayal of the Cumans, the people that had done the most in repelling the Mongols. Some of the barons went to Köten’s house with the intent of killing him as scapegoat or handing him over to the Mongols, possibly believing the Cuman–Kipchaks were Mongol spies. However, the barons had Köten assassinated in Pest on 17 March 1241.[93][c] When news of this outrage reached the Cuman camp there was an eruption of “Vesuvian intensity”. In revenge for this victimization they slaughtered a vast number of Hungarians.[11]: 117 [13]: 22  The Cumans then left for the Balkans and the Second Bulgarian Empire, going on a rampage of destruction through Hungary “equal to that which Europe had not experienced since the incursions of the Mongols”.[15]: 37 [94]

With this departure of its only ally and most efficient and reliable military force,[26]: 43 [95] Hungary was now further weakened to attack, and a month later it was destroyed by the Mongols.[15]: 186 [64]: 173  After the invasion, King Béla IV, now penniless and humiliated after the confiscation of his treasury and loss of three of his border areas, begged the Cumans to return to Hungary and help rebuild the country.[15] In return for their military service, Béla invited the Cumans to settle in areas of the Great Plain between the Danube and the Tisza rivers; this region had become almost uninhabited after the Mongol raids of 1241–1242.[96]