I was confused about this myself - people were saying that it arose from mocking sounds made by people with certain intellectual disabilities, but I couldn't remember anything that hinted at that context - but... Let's just say that if you run a Google image search on "derp" or "herp derp", the evidence in form of images photoshopped to look like people with Down Syndrome as well as stuff that I'm not sure how to describe but let's just say it's pretty clear what is being referred to, hits you like an anvil of ableism.