I understand vertical and horizontal straight lines as borders since they can follow latitude and longitude lines. And then many borders aren't straight because of following natural features.
But wouldn't a diagonal straight line be more difficult to compute and verify? How common were they?
The border follows the basic trend of the Sierra Nevada mountain range. The diagonal part of the border cuts through terrain that was - and still is - largely uninhabited. There was a survey problem that occurred in the case of Aurora, a boom town that served as home of Samuel Clemens in 1862 before he traveled to Virginia City and took the name Mark Twain. The location of Aurora, which is just inside in the Nevada side of the line, was not clearly understood. Residents ended up voting in both Nevada and California elections since both states claimed the community, even though it served as the county seat of a Nevada county (Esmeralda County). A subsequent survey demonstrated that Aurora was part of Nevada. This, however, was a rare event in a largely uninhabited region.