Check your electrodes. This only can happen if wells (where you load your sample) are nearer to positive electrode. The proper orientation should be your wells would be nearer to the negative electrode and DNA will migrate towards positive electrode through the gel.

Thanks Deba..I actually used the proper orientation. The migration happened but actually went above the well height created by the comb. You can see that on image

Most probably the anodes and cathodes were oriented in opposite directions. Please check if the orientations were like Red- Red and Black- Black while running the Agarose gel.