More often than not, this issue is due to a seal leaking air into the unit.
There are a few possibilities that would create frost build up:
Check the seals around the freezer. Be sure they are sealing properly. If they are not that means that there is warm are getting into the freezer and that would create frost build up. Place a dollar bill in the crack between the door and the gasket, as you move around the seal if the dollar moves freely there is a seal leak.
Door being left open allows warm moist air to enter condensing on the walls and around door opening forming frost and / or ice droplets.
Hot foods produce large amounts of moisture (steam). Moisture naturally condenses on the first cold surface it touches. When moisture contacts frozen surfaces, it manifests itself as "frost" (tiny ice crystals).To prevent frost buildup, hot foods, especially large pots of soup / stew should be cooled to room temperature before placing in freezer.
If your frost build up is solely on the bottom you may also have a clogged drain tube. You can clean this tube by accessing the back of your refrigerator.
If none of these tips help, I recommend scheduling service so a technician can properly diagnose and repair your freezer.