A spinning top process (or rotate) about its central axis due to the lateral forces given initially or due to lack of speed (as during the end of spinning). When the speed becomes too low (not zero) for it to hold it (its mass) upright, it falls down.
As the body of the top touches the ground, due to friction it is unable to spin and so the remaining spin (force) forces it to roll on the ground, obviously in the opposite direction.
Due to tapering shape of the top, it tends to rotate along a circle in the reverse direction, with the nail pointing towards the centre.