One of my air conditioners has the same issue with ice forming near the coils and requires defrosts. I haven’t figured it out yet but I can offer you some possible ideas.
# It could be a defect in the thermostat, causing the compressor to work at full power constantly, leading to frozen coils.
# A. Condensation or water collecting somewhere it shouldn’t. If water is near the coils it freezes. This problem then starts to compound, as the frozen coils then start to prevent air flow throughout and the chunks of ice grow larger. It gets worse until the compartment turns into a self-contained chunk of ice that must be defrosted. Checking to see if the fridge is level may help. If it sags in a certain direction that could potentially prevent water buildup from draining.
# B. Air flow restricted somewhere or fan malfunction. If the fan is not working properly, the air will not circulate where necessary. Also, proper fan function helps prevent excess humidity in the compartment. These things contribute or can be the cause of problem A.
Some band-aid temporary solutions that can help might be putting spray wax or some other type of hydro-repellant on the coils. Or a moisture absorber like salt around that area. Or putting a small fan or an appliance that tends to generate excess heat near refrigerators vents. Note that these probably won’t fix the problem but it can minimize it.