Github feature branch made branches automatically and merged it again

Question

If you see the picture below, the blue branch is a feature branch Dev A and Dev B are working on. No one has branched off of the feature branch. But when Dev B removed some files, committed and pushed the changes to repo, it was created as a branch of the feature branch in git client graphical view (sourcetree). When Dev B checks git status, it still says the working branch is the same feature branch.

Dev A created some service class, committed and pushed it. It shows it still is in the feature branch. Why does this exactly happen, anyone can help explain please? Is it just graphical? Or either Dev A or Dev B didn't pull a change in between. Additionaly, on next git push by Dev B, the red new branch merged back to the feature branch.

Upon request, here is the git log of Dev B:

* af335d1 added...
*   63fa2a2 Merge branch 'feature-branch' of github.devops.abcCorp.local:namespace/project into feature-branch
|\  
| * c8d62c1 removed...
* | 74456c5 created...
|/  
* b321f9a fixed...
* 97d7c33 added...

Answer 1

What happened is Dev A tried to push, but it got rejected, so (s)he pulled and then pushed successfully.

A pull is equivalent to a fetch and a merge. For example, if you are on feature_branch:

git pull origin feature_branch

is equivalent to

git fetch
git merge origin/feature_branch

The reason there's a branch in the log is that, since Dev A's local branch diverged from the remote branch, a fast-forward merge was not possible and git had to create a merge commit.

You can avoid this by using git pull --rebase to reapply your local commits on top of the remote branch.

              |  local feature_branch           remote reature_branch
--------------|------------------------------------------------------
before        | A--B--C                        A--B--D
              |
pull          | A--B--C--M
              |        \ /
              |         D
              |
pull --rebase | A--B--D--C'