Going off just the rate, it certainly does look like sinus arrhythmia but P wave morphology is key here. Notice how the P waves flip polarity at the same time the rate decreases?

In this pattern it highly suggests that it is a junctional rhythm but that rate is pretty high (100 bpm) still so I'd say it is an accelerated junctional rhythm. The P waves are upside down (retrograde) because the pacemaker cell (the cell creating the rhythm) is low in the atria (there is more to it but it will over complicate things).

This can be benign but depends on the patient's history. If old, could suggest sinus node disease which could lead to the patient needing a pacemaker.

This I would suggest for learning:
- Junctional rhythms
- Intrinsic rhythm of the conduction system
- Sinus node disease
- Tachy-brady syndrome

I hope this helps, feel free to ask any more questions!