How do brown dwarves form?
  We review and evaluate four mechanisms for forming brown dwarves: (i) dynamical ejection of a stellar embryo from its placental prestellar core; (ii) opacity-limited fragmentation of a shock-compressed layer; (iii) gravitational instabilities in discs, triggered by impulsive interactions with other discs or naked stars; and (iv) photo-erosion of pre-existing cores. All these mechanisms can produce free-floating brown dwarves, but only (ii) and (iii) are likely to produce brown dwarves in multiple systems, and (i) has difficulty delivering brown dwarves with discs. 
Anthony P. Whitworth & Simon P. Goodwin