Latest commit will look like detached in Commits Graph if the latest commit has a timestamp earlier than its previous commit.

It could happen if the computer which the previous commit (P) is made from has a wrong time (say off by several hours or a day), and the latest commit (L) is made from another computer that has the time properly synced. Now the commit P would have a timestamp like 6/22/2015 13:00, and the commit L would have a timestamp like 6/21/2015/17:00.

Theoratically, the commit graph should follow the exact relationships between commits, ignoring everything else. For example, command "git log --graph --decorate --pretty=oneline --abbrev-commit" will show the graph correctly. However, at least in SourceTree for Windows, the commit graph will not have commit L listed above P. Instead, commit L looks like totally detached from any previous commit.