【Accidental】 , 【fortuitous】 , 【casual】 and 【contingent】 all mean happening by chance.
【Accidental】 stresses chance or unexpected occurrence.
【Fortuitous】 so strongly suggests chance that it often connotes entire absence of cause. (see also: 【Fortuitous】 vs Fortunate )
【Casual】 stresses lack of real or apparent premeditation or intent.
【Contingent】 suggests possibility of happening but stresses uncertainty and dependence on other future events.