Something I realized while watching a documentary, most of the videos or photos I see of tanks, on either side in WWII, that have been destroyed or knocked out have their guns facing downwards. Why did this happen?
Towards the middle of WWII most tanks in the field were upgraded variants of something that was already in service at the start. One of the easiest ways to improve the effectiveness of your tank was by making the gun longer, which for example was what the Germans did with their Panzers III and IV. However, that adds a problem, since the longer barrel will also be heavier and thus make it more difficult for the gunner to raise the gun, as powered elevation was not a thing yet.
The most reasonable solution would be to move the trunnions so that the gun is balanced again, but this is hard to do when you have a limited amount of space to work with in the turret. One answer is a counterweight, but this just adds more weight to a tank, which is something you really want to avoid doing.
There was one clever alternative: a spring of some type. The Panzer III had a torsion bar that braced against the roof of the turret to help balance out the gun, the Panzer IV had a big vertical spring that braced against the floor of the turret basket.
Extreme temperature from fire will alter the properties of the metal and the spring will deform. Without anything helping to keep the gun up and the elevation mechanism also weakening from the heat, it will sag. You can also see this effect on burned up tanks with torsion bar suspensions: their idlers will almost be touching the ground because the suspension doesn't work anymore.
Many tanks also had a way to disengage the elevation mechanism and lock the gun in place during travel. If the mechanism was disengaged when the tank burned and the travel stop fails, the gun will fall down since there is also nothing holding it up anymore.
Sources:
P. Samsonov Doroga na pensiyu https://warspot.ru/13323-doroga-na-pensiyu
P. Samsonov Spetsialnaya chetverka https://warspot.ru/14157-spetsialnaya-chetvyorka
Library and Archives Canada RG 24 C 2 (Canadian Military Headquarters, London) reel C-5775 slide 3768
Library and Archives Canada RG 24 C 2 (Canadian Military Headquarters, London) reel C-5775 slide 3401
Library and Archives Canada RG 24 C 2 (Canadian Military Headquarters, London) reel C-5779 480