It doesn't appear there's a root flare at all. That can come from one of 2 issues. Either it was planted too deeply, or it suffered from frequent damage at the base (aka "mower blight"). You can check for the former by digging down until you find your first significant horizontal root. They should originate just below the ground at 90 degrees (not circling around). Even being off by a matter of 3-4 inches will typically result in this outcome. The tree will look healthy for maybe 10 years, but then the root system can't keep up and there's no buttress strength, so it will just snap off at the ground.
