The other answers are correct that you are seeing the normal defrost cycle of your freezer. I'd just like to add two additional observations:
1) There are commonly two kinds of defrost timers: mechanical and adaptive. Your fridge seems to have a mechanical defrost timer due to its regular 14hr cycle regardless of usage. That means it's often wasting energy defrosting when it doesn't have to.
Adaptive defrosters OTOH try to be more efficient and not run so frequently. They have a computer monitor things like how often the doors have been open since last defrost, how long the compressor has been on since last defrost, how long the heater was on during the last defrost, etc.
(Unfortunately, that's not how it necessarily works out in practice. For my GE fridge, at least, the circuit board has been through many revisions and still fails quite often...and it's darn expensive to replace, too, compared to a simple timer! It's a neat idea, but there's clearly more work to be done in this area. It doesn't take too many CPU - and food - replacements to outweight the minor savings of the defrost cycling running a little more often IMHO.)
2) It looks like you're directly measuring the air temperature in the freezer, so you're catching every little fluctuation. The actual food in your fridge isn't changing so rapidly, so more accurate temperature monitoring for things like food, vaccines, etc. usually involve a probe with a temperature buffer of some kind to better reflect the slower temperature changes that the items in the freezer are actually experiencing (e.g. a probe bottle filled with glycol). And while the claim that a fuller freezer is more energy efficient than an emptier one is still a bit controversial, from a temperature stability standpoint, a fuller freezer should have a more stable temperature, too.