Many of us like code formatted in a particular way. Many projects also enforce a particular indentation style. Neither may match KDevelop's default indentation style. However, this can be customized: Go to the → menu item, then click on Source Formatter on the left. You can choose one of the predefined indentation styles that are widely in use, or define your own one by adding a new style and then editing it. There may not be a way to exactly re-create the style in which your project's sources have been indented in the past, but you can come close by using the settings for a new style; an example is shown in the two pictures below.
With KDevelop 4.2.2, you can create a new style for a particular mimetype (e.g. for C++ header files) but this style does not show up among the list of possible styles for other mimetypes (e.g. for C++ source files) although it would of course be useful to use the same style for both kinds of files. You will therefore have to define the style twice, once for header and once for source files. This has been reported as KDevelop bug 272335.