I can see how the new usage got started, but I think that the meaning has changed enough that it is a new meaning that’s separate from the original meaning.
It seems likely to me that the people saying “Well that was random!” don’t know what the word random really means (or used to mean) and are using it as if it meant “surprising”. They don’t mean “random coincidences can be surprising”, they mean “that was surprising”, whether it was coincidental (random) or not.
The kids here use “random” to mean a comment that is “non sequitur” or inappropriate
I always thought a “sketchy” person was somebody who looks as if their portrait would be accompanied by the caption “Police sketch artist’s rendition” 
The OED says that the “inexplicable, unexpected” sense of random is “(Orig. U.S. Computing)” (and it has cites going back to 1971). It doesn’t have the “seedy” meaning of sketchy at all.
I myself have never heard the “seedy” sense of sketchy in Ireland or Australia, the two countries I have lived in, and I think that in both countries the “aimless, haphazard” sense of random is very much dominant over the “surprising” sense, which is still seen as colloquial.
I think I have heard the “seedy” sense for sketchy here though. There’s also “Sketch!” and to keep sketch but they seem unrelated.