MBB2256 Maytag bottom freezer occasionally frosts over

[bshust]

Roughly once every 3 months this happens to the fridge at my church.  Works fine again after defrost.  Note that the freezer is often kept quite full...at the time of this photo, there were 17 frozen 5-lb pans of macaroni & cheese taking up most of the volume in the freezer.  For several weeks, I take a fridge and freezer temperature reading every few days; usually I find it OK -- about 34-40 degrees in the fridge, and -6 to 0 degrees in the freezer.  Then seemingly overnight, the fridge is 60-degrees and the freezer is still below zero.  The drain is not frozen over.  How likely is it due to the large amount of frozen food, versus a bad sensor or mechanical defrost timer?  Chicago area.