why the magento is generating this custom redirects in URL Rewrite Management.
If you have a configurable/simple based catalog, this is a common issue in Magento 1.* (I think it is fixed since some version of 1.9)
Take a look at this
Basically, it has no sense to assign an url to non visible products, as it typically happens for simple products, when they are a configurable child
If that's not your case, then it should probably start to avoid url duplication (more than 1 simple product has the same name, so the same url) in your catalog, and reindex process adds new redirects every time it is executed. Take a look at this then