Peanut butter contains salt which is an electrolyte which can lead to the sparking in the microwave.
Another reason that peanut butter sparks when microwaved is because the Peanut Butter has a high carbon content and as such as a high degree of electrical conduction which "Shorts" out the microwaves as they pass over the peanut butter.
That can lead to the sparking of the peanut butter when you put it in the microwave to melt it.
It's usually nothing to worry about and shouldn't damage your microwave or cause any other issues.
I've microwaved peanut butter and it sparks at times and my microwave has never suffered any damage.