From Longman Dictionary of Contemporary Englishfall in phrasal verb1 FALLif the roof, ceiling etc falls in, it falls onto the ground SYN collapse2 PMLINEto start walking or forming a line of people behind someone else behind His men fell in behind him. → fall→ See Verb table
Examples from the Corpus
fall in• During the hurricane the roof fell in.• The government is worried about the recent fall in house prices.• We need to fix the ceiling before it falls in on us.fall behind• Caroline sighed and fell in behind him.• Ritchie quickly handed him his three wads of money and walked away, the rest of the posse falling in behind him.• The Tories have responded to calls for unity and fallen in behind John Major.• A few shook themselves loose from the tangle, and pounded onward, others skirted the fallen and fell in behind them. 