One thing to realize about such information related to mythology is that there are hundreds of folk-stories that have got merged from different branches and sources. Not all of them might be true, or accurate. Nevertheless, they should offer a plausible explanation.
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The aforesaid incident is possibly referenced to the event, when after Lord Krishna departed, leaving his mortal body, Arjuna was tasked with taking the wives of Sri Krishna from Dwarka to Hastinapur.
Along the way, Abhira robbers attacked the convoy, and Arjuna decided to kill them all. But surprise, Arjuna was not able to recall the mantra (sacred hymns) required to wield the divine weapons to defeat a large number of attackers. However hard he tried, Arjuna failed to recall the hymns. He was not able to wield his bow Gandiva, either. As a result, a number of wives of Sri Krishna were kidnapped by robbers. Some of them were able to free themselves and some of them self-immolated. It is also referenced that a lot of wives had to suffer this fate due to a curse of Sri Krishna after the wives had been unfaithful (beginning of Kaliyuga!).
The reason for this could be that one must realize that God and devotee are one. Arjuna and Krishna were Nara (man) and Narayana (God). Without Narayana, there can be no Nara, and vice-versa. The greatness of Arjuna came from the fact that Krishna resided in him. Krishna saved him from possible defeats and death multiple times and was his constant companion. So when Krishna left earth, he left Arjuna’s body too and did not reside further in him. So without Krishna, Arjuna had no divinity left, and so he was not able to wield any divine weapons and forgot most of his knowledge.