I've known people that start with cold and hot water. I like to start with cold, because it's the same as boiling potatoes. If you start with hot water, your vegetables will start to come apart on the outside while the inside is still not cooked through. Making stock is a slow gentle process to extract the most flavor. If you want to rush it, try making it in a pressure cooker. This is what I do because I don't want to make the utilities bill too high. It works fine and only needs to cook at full pressure for 20 or 30 minutes and let it cool on it's own accord.
Most hot water comes with an unfavorable taste, for me it is the metallic interior of the hot water heater/boiler in my building. When you start with cold, you don't get that taste.
It's chemistry. The object of stock-making is to extract flavors from the ingredients and put them in solution, not to cook the ingredients themselves. Various compounds dissolve at different temperatures. Starting with cool water allows the maximum extraction, from those that dissolve at lower temps to those that dissolve at higher temps. Further, if you were to "cook" the ingredients quickly by s
tarting at a high temp, you are just locking the flavors inside the ingredients rather than letting them flavor the solution.
I think it's a combination of both the metallic taste of much hot water AND the desire to extract the most possible flavor from the solids you are heating in that water.
Another reason is so the stock doesn't become cloudy.
Using cold water to start your stock will form larger protein aggregates, which will later stick to the edge of the pot or float to the top which you will later skim. If a stock is started with hot water, the proteins will coagulate faster, making smaller protein particles, causing your stock to be cloudy