Water can enter a refrigerator intentionally through an icemaker connection or as a result of daily use, in the latter case water moisture carried in the air infiltrates everytime the doors are opened, or if the door gasket seal is leaky. Once inside the moisture is collected on the cooling coil as frost. As the frost builds up and the refrigerator enters in defrost mode the is frost melted, the resultant water then drains outside the cabinet in to a drip pan.
Your problem is that you have a blocked drain tube. Clear the blockage and the problem should disappear, also verify you dont have excessive infiltration by checking for condensation inside the cabinet,(around the gasket sealing surface and the top of the cabinet)