Time machine can keep local snapshots. While this feature is useful if you travel and don’t have access to your Time Capsule, the local snapshots can consume a lot of the local disk space in /.MobileBackups. There is a complex Removal Algorithm for old local snapshots based on the remaining free disk space.
To find out how much space your local snapshots consume (Local Snapshots):
Apple-Logo → About This Mac → Further Information → Storage: Backup == Local Snapshot Space
Alternatively from commandline: test -d /.MobileBackups && du -hcs /.MobileBackups
It is useful to control the behavior of the local snapshots yourself (Controlling Local Snapshots) using tmutil disablelocal and tmutil enablelocal.