Here is a short program that went into this weirdo mode several times
fairly quickly.
Adding #include directives might be involved, but maybe not.  Again,
the same symptom:
everything goes flush left until I close out and re-visit the file.
So, when you first visit
the file, I am sure it will indent properly.  And just because you
fiddle it, it doesn't mean
it will stop indenting properly.  I do not know what the trigger
mechanism is.  I just
know it was much more frequent (several times in a few minutes).

Again, "linux" style:

$ cat ../.dir-locals.el

(
 (nil . ((indent-tabs-mode . t)
         (tab-width . 8)
         (show-trailing-whitespace . t)
         (fill-column . 132)))

 (c-mode . ((c-file-style . "linux")))
)