There are a few possibilities. As others have said, your DC voltage is fluctuating; the question is why.
Some multistage converters change their output voltage periodically per their programming. Progressive Dynamics with the charge wizard will (once the charge sequence has completed) jump from 13.2V to 14.4V for several minutes about once a day.
Some converters have wonky or varying output due to faults in the converter, and these should be replaced.
Sometimes there are self-resetting circuit breakers in various parts of the DC system, and a high load or a failing circuit breaker or a poor connection at the breaker will cause these to cycle on and off. Loose wires or connections can have a similar effect.
If you have a large varying electrical load, the voltage of the system will vary due to it. An inverter powering some nonuniform load is one example. Even a moderate varying load can often cause voltage fluctuations for the other things on the same circuit.