i read this on reddit like a looong time ago. basically, scanlators (non-MTL ones) spend a lot of time working on a series, so when someone suddenly uploads it too, it feels like their work is being taken over. its called sniping, afaik its like an etiquette on scanlations
the problem is that sniping doesnt guarantee faster or better updates. sometimes the new scanlator is inconsistent or low quality, and the original scanlator loses motivation to continue, so readers end up with no updates at all. if the original scanlator has abandoned the series, though, most people dont mind someone else picking it up
Well, if you are doing a task every week, but sometimes when you show up to do it there is someone else who already did it, the prep-work and the journey to the task is wasted. I'm guessing it can be pretty annoying to have everything set up and then not doing it and having to do it again every week just in case. Many uploaders even ask uploader Friends to take over their uploads if they know they won't be able to do it for a time.
They aren't getting paid, but that don't mean they don't commit
On the other side, I've been following a couple of series with multiple uploaders at the same time and it's also annoying on the reader's side(〜 ̄△ ̄)〜

why do uploaders not want others to upload over them? from what i know, they don't earn money outside of donations so i've always wondered why uploaders seem to fight over who should upload a series. some series are even reserved with notices before it's officially translated—but why? isn't it better if someone else is able to upload whenever an uploader can't do it as it takes the load off of them?