Maharaja Yudhisthira requested Narada to explain (the incident of Sisupala's receiving sayujya-mukti instead of entering hellish life) the cause for such a wonderful event

Expressions researched:
"Mahārāja Yudhiṣṭhira therefore requested Nārada to explain the cause for such a wonderful event"

Srimad-Bhagavatam

SB Canto 7

The śāstras enjoin, tad-vijñānārthaṁ sa gurum evābhigacchet: [MU 1.2.12] when one is perplexed by the difficult problems of life, to solve them one must approach a guru like Nārada or his representative in the disciplic succession. Mahārāja Yudhiṣṭhira therefore requested Nārada to explain the cause for such a wonderful event.

SB 7.1.20, Translation and Purport

This matter is undoubtedly very wonderful. Indeed, my intelligence has become disturbed, just as the flame of a candle is disturbed by a blowing wind. O Nārada Muni, you know everything. Kindly let me know the cause of this wonderful event.

The śāstras enjoin, tad-vijñānārthaṁ sa gurum evābhigacchet: [MU 1.2.12] when one is perplexed by the difficult problems of life, to solve them one must approach a guru like Nārada or his representative in the disciplic succession. Mahārāja Yudhiṣṭhira therefore requested Nārada to explain the cause for such a wonderful event.