"To stop the momentum" means to halt or slow down the progress or movement of something that is gaining speed or making significant progress. Full definition
Everyone wishes to spend a happy and healthy life and any severe illness can stop the momentum of your life. (policybazaar.com)
Maybe not, but it's hard to stop the momentum when someone of Indian heritage in any community makes a request for the change to be made. (sportstarsmag.com)
It wouldn't make much sense to potentially stop the momentum gained from the team's best display this season. (latestchelseanews.com)