It should be pretty clear from "Comma before etc.?" that there should be a comma before etc. when used in a list, but very often I see the opposite. What could be the reason?
Here are examples from Wikipedia and Wiktionary:
... according to Red Army directive, fuel, equipment, railroad cars etc. were similarly concentrated there. Context, Wikipedia article.
In the analysis of musical form, sections, units etc. that can be defined on the time axis are conventionally designated by letters. Context, Wikipedia article.
Locally, this may refer the leading crop of a region, such as oats in parts of Scotland and Ireland, wheat, barley etc. in England and Wales. Context, Wiktionary article. (Note that there is another error in that sentence, a missing word.)
Update 2: added example from Wiktionary.
Update 1: change now applied at the source for the first two examples.