1
general::
Fall out of line or into disorder; also, fail to conform, deviate. For example, The recruits were warned that they must not break ranks, or Harry was told to adhere to the party platform and not break ranks. This idiom uses rank in the sense of "soldiers drawn up in line," and the term originally referred to their falling into disarray. The figurative usage dates from the mid-1800s.
American Heritage Idioms