There must be something just out of view that it flipped over. It is also possible something flipped it, such as Godzilla or Mothra...
In the 90's for some reason the Army thought it would be a good idea to see how easy it would be for a bulldozer driven by a combat engineer to flip a tank out at NTC. Unlikely, but it is possible this is the aftermath clean up. Stupid idea anyway.. what are the chances of an unarmored bulldozer getting close enough to a tank without being shot to do something like this?