WordNet (r) 3.0 (2006):

falling out
    n 1: a personal or social separation (as between opposing
         factions); "they hoped to avoid a break in relations" [syn:
         rupture, breach, break, severance, rift, falling
         out]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "falling out":
   alienation, altercation, beef, bickering, breach,
   breach of friendship, break, cleavage, cleft, controversy,
   disaffection, disfavor, dispute, disruption, disunion, disunity,
   divergence, dividedness, division, estrangement, feud, hassle,
   open rupture, recall of ambassadors, rift, row, run-in, rupture,
   schism, separation, split