First, in your citation 1, you have the generational aspect of "on" and "by" reversed. The original and correct[1] usage is "by accident". Acceptance by the younger generation doesn't make "on accident" correct, it is just accepted for lack of knowing otherwise. (This will, granted, eventually result it in showing up in dictionaries as a usage.)
The new generation, whether less read or less likely to have read the writings of prior generations, is less influenced by existing usage, and mistakenly verbalizes "on accident" to over-regularize with "on purpose".
"Over-regularization" is the kind of mistake a toddler makes until they learn correct usage by hearing and reading correct usages from multiple example experiences.[1] As the new generation reads less old material, and socializes textually with peers more and earlier, incorrect usages imprint to the point they gain defenders from the "everyone's doing it so don't call it wrong" camp.