Sometimes the fridge goes through a defrost cycle, where the blower and the compressor goes off and a heater comes on in the freezer. By this it melts the frost off the evaporation coil and the water runs down in the back of the freezer.

Sounds like drain tube is plugged so that the water from the defrost cycle cannot drain into the drain pan under the unit.

I would check that it is clear.
remove the cover in the freezer so you can get at the evaporator coils. The drain should be directly under the coils.
If plugged a turkey baster with warm water, an air compressor with low pressure or a wire can often be used to clear it.

Also check that it is not plugged with dust etc. at the drain pan end.
You might as well clean the condenser/compressor fan and the condenser coils while you are down there.

If it looks like stuff is growing in the tube I would flush it with a 10% bleach solution. This should kill the stuff and let you flush it out. You may have to do this several times, let it sit for 15 minutes or so between flushes.

Be careful that the drip tray does not overflow and empty the drip tray after you are done.

Be sure to unplug the unit while doing this.

Check that the kick plate and rear vents are clean.
Check/clean the condenser coils under the unit.
Check that the condenser/compressor fan runs when the compressor is on. Also that it's blades are clean.
Doing the above makes the unit more efficient but also help the defrost water evaporate out of the drip tray.

I am not sure how much water there was so cannot say if it is just from defrost.

I would check the door seals. Any leakage past the seals will put warm moist air into the unit. This will then condense on the evaporator coils causing more water than what would be normal.