Tumblr in general has a thing for seeing itself as superior to deviantART (hereby referred to as dA). While much of their reasoning for it is their supposed superiority at sourcing (which… isn’t always true…), a large portion of it has to do with ridiculing the relative immaturity of dA. This is apparent in other mockeries of dA, such as screencaps of dA comments:
This account recently had a small tumblr-based meme around it of taking various comments he wrote and reblogging them, the account has since been suspended.
I bring this example up because I feel that it helps to show Tumblr’s attitude towards dA as a whole – full of preteen kids that don’t understand how the world works and what plagiarism is. This is likely related to why the phrase appeared on tumblr and is now used to ridicule dA.
It’s important to note that searching Google for “before you say i am stoling this art let me explain you a thing” site:deviantart.com brings up no results. While it could be a deleted comment, or perhaps Google does not look at dA comments, I have to wonder whether a tumblr user captioned a dA user’s defense of their plagiarism with the phrase, which then went viral. However, if someone does have an OP that would be great.