Two likely possibilities that I have seen over the years:
This used to be quite common on GE side-by-side refrigerator/freezers, but maybe on others as well. Frost-free freezers aren't truly frost free. They create frost and them melt it away, it then drains into a pan somewhere to evaporate. If the drain hole clogs, ice builds up in the bottom of the freezer.
When the gasket wears out, moist air can get into the freezer. The moisture condenses and then freezes on the bottom. If this is the problem replacing the gasket is the solution - usually fairly easy but also a relatively expensive part and specific to each model because it has to be exactly the right length & width.