Having no cause or apparent cause:
— Fortuitous encounters--strange accidents of fortune.

synonyms: causeless, uncaused.

Roget 6: derived from without; objective; extrinsic, extrinsical^†; extraneous etc. (foreign) 57; modal, adventitious; ascititious^†, adscititious^†; incidental, accidental, nonessential; contingent, fortuitous.    implanted, ingrafted^†; inculcated, infused.    outward, apparent etc. (external) 220.    ... show more

Roget 156: casual, fortuitous, accidental, adventitious, causeless, incidental, contingent, uncaused, undetermined, indeterminate; random, statistical; possible etc. 470; unintentional etc. 621.    ... show more

Roget 621: fortuitous etc. 156; unintentional, unintended; accidental; not meant; undesigned, purposed; unpremeditated etc. 612; unforeseen, uncontemplated, never thought of.    random, indiscriminate, promiscuous; undirected; aimless, driftless^†, designless^†, purposeless, causeless; without purpose.    possible etc. 470.    unforeseeable, unpredictable, chancy, risky, speculative, dicey.    ... show more

Occurring by happy chance.