Claudia,
There are only a few isolated instances where Project "decides" to do
something but indenting and outdenting are not one of them. If the line
is now a Summary line, it must have subtasks under it. First, "explode"
the file to show those subtasks. Then select them all and outdent them.
Another case where you could have ended up with an unintentional summary
is when creating a master file with inserted subprojects. However I
doubt this is your situation.
The most likely causes of a subtask becoming a summary line (other than
the user knowingly changing it as Steve suggests) is an inadvertent
mouse click or when dragging and dropping a task to another part of the
file.
Hope this helps.
John