Problem: Abandoned tanks becoming destroyed without any reason...how?

[Mobhack]

Wrecks are destroyed if the other side occupies the same hex for 1 turn.

Abandoned tanks (that still have a surviving crew) or other crewed elements are converted to destroyed if that side loses the game, ie it is not a draw, at the end game.

Your knocked out Stug - if the crew is destroyed then naturally a brand new crew and leader was assigned from the training depot, with no kills accumulated for them as yet.