Ice build up in bottom of freezer
Check the ice maker. sometimes they get cracked a will leak water into the freezer compartment. Also check the ice maker fill tube for leaks or someone could have spilled something. Although this is tipical of a clogged drain. What happens is when the fridge go into derfost, it will melt all the frost that is on the evaporator, turning it into water. The water drains down to a trough under the evaporator and down a drain hole in the trough ( this trough is behind the evaporator cover ) Removing the melted frost from the freezer. If that drain is clogged, the water cannot leave the freezer. Then when the fridge starts back up ,it will freez the water that was not drained. After repeated defrosting the water will fill this trough and run off into the bottom of the freezer to be frozzen when the fridge starts to cool again.