A model number would be helpful here.
Find your model number - AppliancePartsPros.com

With luck I will be able to find a wiring diagram.

How often does it happen?
Defrost would happen every 6 hour (absolute minimum) to every couple days.
It all depends on how the defrost is controlled.

Is this unit new to you?
Has it done this from day one or has the unit worked properly in the past?
The reason I ask is that sometimes folks modify a unit to do a quick fix to a problem and sometimes they create other problems.

I cannot find any info on the given model number.
Could be that it is just too old.

I did find a HMG191477 at the Sears parts site.
It was for a freezer on top unit and did show a wiring diagram.

It did have the fans wired in parallel with the compressor.
The main control thermostat was wired to one side of the compressor, the fans and the defrost timer which was an 8 hour timer.
So the defrost should happen every 8 hours of compressor/fan on time.
It would be nice to know if yours is wired like this.
Then the 7 hour recurrence time would be off for a defrost cycle unless the compressor is running continuously which it should not be doing.
Note that there are units out there that just do a defrost every 6 or 8 hours independent of compressor run time.

The other side of the fans and compressor went to the defrost timer relay.
So when in cool they are all on.
When in defrost they should all be off.

Sorry I cannot be of much use here as I have never seen or even heard of your problem/symptoms.
Check on the back of the unit for the wiring diagram.

Is it possible that the compressor is overheating and kicking out?

If it is possible.
Clean the condenser/compressor coils.
Check that the condenser/compressor fan is running correctly and it's blades are clean.
Check/clean the front kick plate vents and the rear cover vents.

Try to catch it when it does this at the start and then see if the defrost heater is coming on. Be careful that you do not burn your fingers.
One thing to note is that the defrost thermostat must be frozen so you have to get it at the start of the defrost cycle. It opens just above freezing and kills power to the defrost heater.