Normally there will be reverse leakage when a diode is reverse biased. Due to the increased potential, holes on the n side of semiconductor are dragged to negative terminal of the battery and electrons on the p side is dragged to the positive terminal of the battery. This constitute a reverse a current which is a also called as reverse saturation current.
This is because even though increase in reverse potential, current will not be increased that much for a certain limit. This leakage current varies with diodes according to the spec sheet.