It most likely is the defrost drain being plugged up. remove the lower shelves of the freezer, and at the bottom of the back wall is a drain hole, probably iced over. Break up all the ice on the freezer floor and under the back cover. get some good hot water and a turkey baster and force water down the drain hole till it is free of the clog. Over the years the drain tube gets a buildup of algae like stuff and blocks the drain.