Unassisted triple plays usually occur when the runners are moving with the pitch.
What probably happens here is that both runners are going. Second baseman moves to cover second base. Batter hits a line drive or a bloop that the first baseman catches. First baseman tags out the runner, who is caught between the first baseman and second base. First baseman continues to second, where he gets the force out.
If the first baseman caught the fly/liner, then tagged first base, he would certainly have throw to second, eliminating the unassisted TP.