LB: I highly recommend reading about the incident directly from the text of the fifth chapter of sefer Yehoshua.
The issue has to do to with the korban Pesach – also having been performed only once following the exodus from Egypt – that requires participating males to be circumcised.
Interestingly, Rash”i in Devarim (33:9) quotes the Sifrei that the Leviim did perform bris milah in the desert, unlike their cousins.
The Talmudic reference is masseches Yevamos, 71b & 72a.