I don't recall any federal rule change necessitating early negotiations.
I do recall UPS wanting the Contract wrapped up early to avoid bad publicity. And to have it ratified before the end of the year 2007, so their Withdrawal Liability would be limited to $6.1 billion, and not include the additional billion(s) that they would owe if they withdrew normally at the Contract expiration date of July 31, 2008.
Hoffa and Crew, of course, moved Heaven and Earth to make it happen.