SourceTree hangs periodically for me. Sometimes when pushing to remote. Sometimes when committing local. Sometimes when staging files for commit. Once it even hung when I was clicking on a file to see a diff. It is an indefinite hang and requires a force-quit.

Others have reported that it is when they come back to the app. That isn't always the case for me. It sometimes will work for a minute or so, just long enough for me to be committing or pushing.