It is said there that the Lord destroyed the Yadus who became a burden on the earth. This perception was created by the Lord for Arjuna and others. The cause is explained at the end of the Eleventh Canto. Arjuna here says that the Yadus became a burden on the earth, but actually they were the ornaments for the earth. That is because the Yadus were eternal associates of the Lord. Just as a woman does not feel that her ornaments are a weight, so the earth did not feel the weight of the Yadus. Even the devatas who had appeared in the Yadu family cannot be said to be a burden since they also were without rajas and tamas. Therefore the destruction was a means of having the devatas and the eternal associates return to their appropriate destinations. The Lord himself makes reference to the burden:

kiyan bhuvo ’yam ksapitoru-bharo
yad drona-bhismarjuna-bhima-mulaih
astadasaksauhiniko mad-amsair
aste balam durvisaham yadunam

The burden of the earth caused by the eighteen aksauhinis formed by Drona, Bhisma, Arjuna and Bhima has only been slightly reduced. The intolerable strength of the Yadus remains because of my portions such as Pradyumna. SB 3.3.14