In the larger diameter blood vessels, it’s also due to relieving back pressure and finding a path of least resistance. This causes existing pathways to dilate and eventually join together to form a new channel / anastomosis (aortic coarctation is quite an extreme example - because of severe narrowing at the aorta existing blood vessels widen so much they cause notching of the ribs, which is visible on chest x-ray). Things like liver cirrhosis can cause back pressure and dilatation in a similar way.
