From Longman Dictionary of Contemporary Englishfall back into something phrasal verbto go back to doing something or behaving in a way which you did before I was amazed at how easily I fell back into the old routine. → fall→ See Verb table
Examples from the Corpus
fall back into • Deep inside, a high, wailing voice stopped crying and fell back into a deep sleep.• It can just as easily fall back into anti-feminism.• It would have been very easy for the United States to fall back into its historic isolationism. 