Reasons a product would go on sale:

* Changing packaging design and want to purge old design
* Overproduction occurred that would result in the product expiring before it can be shipped out/sold (same thing for over-ordering)
* Time of year averages indicate that the time period would otherwise be low in sales
* Holidays
* Coincides with special event
* New related product launch (think yearly phone releases)

Feel free to contribute to this list (it's for a future post on the personal blog)