tandem repeat sequence; causes of contraction and/or expansion question
Gene-duplication would not result in contraction (only expansion). Endoduplication would affect all the genes in the chromosome. Error prone repair would cause small-indels but not expansion of the repeats. Inversion will certainly not do anything to the sequence except changing its orientation.

During unequal crossover, the chromosomes can align imperfectly because of multiple alignment sites in the repeat region, thereby leading to unequal crossover products.  Crossing over however essentially happens because of homologous recombination. Crossover is a "phenomenon" and not a "mechanism" in a true sense.






  Mechanism of crossover
  
  Reproduced from: Bugreev, Dmitry V., et al. "The resistance of DMC1 D-loops to dissociation may account for the DMC1 requirement in meiosis." Nature structural &amp; molecular biology 18.1 (2011): 56-60.


Another thing to note is that homologous recombination in the absence of crossover can lead to expansion of repeats but not contraction (see below).