This is actually a common problem among our players, without them knowing what the reason behind it is. It is a feature to prevent players from leaving the battle before the end without consideration for others. So let´s have a look at it.
The server will destroy your tank if you quit from a battle via the menu (Quit to Garage) without being destroyed first. This prevents your tank from moving around the battlefield by itself, or staying put and spotting for your team mates. It is a system that keeps the game fair for everyone.
How does it work?
At the end of every battle, whether it ends by all enemy tanks being destroyed, or the base capped, it might take up to 15 seconds for the server to acknowledge the victory and take it into account. Therefore, if you quit manually to the garage before that happens, your tank will be destroyed (as mentioned above) and grant you a significant penalty for doing so. You can see your tank is deemed "destroyed" by the game if you look at the battle report screen by being greyed out in the red field.
How do I prevent this?
To prevent this from happening in the future, stay in the battle until the game has taken you to the Garage if you are alive. If you are destroyed, then you can leave back to garage without any loss.