Vitamin C oxidises easily and at higher temperatures it oxidises faster. Vitamin C contains a hydroxyketone, -CHOH-(C=O)- which easily forms a diketone, -(C=O)-(C=O)- with O2 (oxygen). It does not physically "break-down". It is an anti-oxidant, which means it reacts preferentially with oxygen, protecting whatever else you have, e.g. lemon juice on apples to prevent browning of the apples which is caused by the oxidation of certain compounds in the apple flesh when exposed to the oxygen in the air. The vitamin C grabs the oxygen before the apple can, thereby changing into the diketone but keeping the apple looking fresh. At higher temperatures this would tend to happen faster and the vitamin C would be "used up" faster.