The ability to veto a trade is available in order to prevent collusion in a league. 
There are two ways to veto a trade, and in League Manager (Private) leagues, this settings can be changed by the LM at anytime during the season:

Public leagues and/or Private league with standard settings: 

• EVERY team manager in the league has the power to cast a vote against a trade, EXCEPT the two teams involved in the trade. Once a trade is accepted, an email will be sent to every team manager in the league with detailed instructions on how to cast a vote. Follow those steps and you'll be all set. 
• If enough votes are cast against the trade (4 out of 10, 50% of teams NOT involved in the trade), then it will be immediately canceled. 
• Teams with more than one team manager will only receive one vote. 
• Once player(s) are traded, the players will be on team's bench until Team Manager manually move them to active lineup. 

Private leagues:

• Unless otherwise changed by the LM, all public league settings apply,
• In LM leagues, LM has the ultimate power to veto a trade or let it go through. 
• LMs are able to see the number of votes for or against a trade prior to the trade being vetoed or processed.
• If the rare scenario occurs where the setting is changed while trade is pending approval, that trade will be grandfathered in to use the original setting that was in place when the trade was accepted.
  Under LM Tools -> Edit League settings -> Transactions and Keepers