In sourcetree 2.3.1 mac os 10.11.6, the application repeatedly hung when trying to reset multiple altered files in one go.
I found that the problem was that I recently changed my settings from 25 lines of context to 6. Sourcetree could not handle that.
I resetted them one by one and changed the setting back to 25.
The files concerned were jsons, used in a dictionary context.