There a number of various reasons why the sink could be leaking, Below are a few possible causes;
Sink combination trap leaking. This could be leaking on any of the joints. We may be able to tighten the joints and put on steel clips instead of the plastic clips. Or we might have to change the whole sink combination trap.
Angle valve/ shut off valve for the water points are leaking. We may have to replace the angle valve/ shut off valve. This may break in the wall and we will need to chisel some tiles/ bricks out to get to the pipe.
The flexible hoses on the mixer are leaking. Some mixers have removable flexible hoses. In that case we will just change the 2 flexible hoses. On those mixers where we cannot change the flexible hoses we will have to replace the whole mixer.
Pvc waste pipe leaking. Either in the cupboard or onto the floor. We will have to check where the leak is and replace the said pipe. This may involve chiseling tiles and brickwork.
Silicone on the sink has come off or wasn’t put on properly. When washing is being done and there is slight spillage some of the water can flow between the sink and counter top and drip into the cupboard. We will need to disconnect the whole sink and clean off all old silicone and re-silicone the sink.