all of the sudden the developer branch is not "marked purple" anymore and is not "branching off" but in line with the master branch (see image http://note.io/PsqqK0)
What happend and how can I fix that?
That is the standard way SourceTree will draw the graph in your situation. A branch will be drawn directly above the branch it was branched from unless/until the source branch receives another commit (you can test this by checking out master and committing anything).
One positive outcome is that anytime a branch is drawn this way, you can expect a merge from development back to master to be fast-forward, unless you check the option to force a new commit.
It should also be noted that the colors of the branches are completely meaningless. They are only user to help visually follow the history of a branch in cases where you have multiple parallel branches.
'Master' is not really a special branch - it's just a kind of naming convention for the main branch. Any branch may be promoted to be the main branch.
See here:: Why does my 'git branch' have no master?
I assume: As any branch might be the main branch, the branch currently checked out is usually drawn as main branch ("straight history line"). This is true from a user point if view: the currently checked out is currently the branch he is focused on ...
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