It's from that deal McCain brokered, in which 7 GOP and 7 Dems agreed that there could only be a filibuster of judicial nominees in extreme circumstances. Basically it hamstrung Frist's threat to invoke the nuclear option, and gave effective control of the confirmation process to 14 "moderates."
McCain took credit for brokering the deal, but Trent Lott was the "moving force". The idea originated with him and all the negotiations and the signing ceremony itself took place in his office -- even though Lott was not one of the signatories.
Lott's sole interest was embarrassing the Senator who had replaced him as Majority Leader, Bill Frist...