SN1 reaction
The carbocation intermediate formed in the reaction's rate limiting step is an sp2 hybridized carbon with trigonal planar molecular geometry.  This allows two different paths for the nucleophilic attack, one on either side of the planar molecule.  If neither path is preferentially favored, these two paths will be used equally, yielding a racemic mix of enantiomers if the reaction takes place at a stereocenter.  This is illustrated below in the SN1 reaction of S-3-chloro-3-methylhexane with an iodide ion, which yields a racemic mixture of 3-iodo-3-methylhexane: