WordNet (r) 3.0 (2006):

crop up
    v 1: appear suddenly or unexpectedly; "The farm popped into view
         as we turned the corner"; "He suddenly popped up out of
         nowhere" [syn: crop up, pop up, pop]

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "crop up":
   appear, approach, arise, be born, become, break out, burst forth,
   chance, come along, come forth, come into being,
   come into existence, come on, come out, come to be, draw on, erupt,
   get to be, have origin, irrupt, issue, issue forth, materialize,
   originate, pop up, present itself, rise, show up, spring up,
   take birth, take rise, turn up