When a frost free refrigerator goes into the defrost mode this is normal
I have a feeling this has been happening rite along and you just noticed it
All side by side refrs have some form of heat between the fresh food and freezer on the mullion ( part you're feeling heat ) this is to prevent condensation from forming because there is a 40 deg. difference in the two sides of the fridge. It should not get this hot. Most newer refrigerators have a refrigerant line running through this mullion instead of an electrical heater. It leaves the condenser and returns to the condenser. If it is getting this hot its due to too long of run times. Remove the back panel of the fridge and make sure the condenser fan is running and / or not obstructed in some way, also see if the condenser coil ( coil under fridge ) is not excessively dirty as this will also cause this line going through the mullion to be warmer than normal.